BUG: read_csv is failing with an encoding different that UTF-8 and memory_map set to True in version 1.2.4
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
df = pd.DataFrame({'name': ['Raphael', 'Donatello', 'Miguel Angel', 'Leonardo'],
                                     'mask': ['red', 'purple', 'orange', 'blue'],
                                     'weapon': ['sai', 'bo staff', 'nunchunk', 'katana']})
df.to_csv("tmnt.csv", index=False, encoding="utf-16")
pd.read_csv(filepath_or_buffer="tmnt.csv", encoding="utf-16", sep=",", header=0, decimal=".", memory_map=True)

```

#### Problem description
This works perfectly with version 1.1.1, but now it doesn't and since this is a nice feature because it removes I/O overhead, I think is good look into this and also because it could break many things

#### Expected Output

```
           name    mask    weapon
0       Raphael     red       sai
1     Donatello  purple  bo staff
2  Miguel Angel  orange  nunchunk
3      Leonardo    blue    katana
```

#### Traceback

<details>

```python-traceback
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "./lib/python3.8/site-packages/pandas/io/parsers.py", line 610, in read_csv
    return _read(filepath_or_buffer, kwds)
  File "./lib/python3.8/site-packages/pandas/io/parsers.py", line 462, in _read
    parser = TextFileReader(filepath_or_buffer, **kwds)
  File "./lib/python3.8/site-packages/pandas/io/parsers.py", line 819, in __init__
    self._engine = self._make_engine(self.engine)
  File "./lib/python3.8/site-packages/pandas/io/parsers.py", line 1050, in _make_engine
    return mapping[engine](self.f, **self.options)  # type: ignore[call-arg]
  File "./lib/python3.8/site-packages/pandas/io/parsers.py", line 1898, in __init__
    self._reader = parsers.TextReader(self.handles.handle, **kwds)
  File "pandas/_libs/parsers.pyx", line 518, in pandas._libs.parsers.TextReader.__cinit__
  File "pandas/_libs/parsers.pyx", line 649, in pandas._libs.parsers.TextReader._get_header
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte
```

</details>


#### Output of ``pd.show_versions()``

<details>

```
INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.8.8.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.2.0
Version          : Darwin Kernel Version 20.2.0: Wed Dec  2 20:39:59 PST 2020; root:xnu-7195.60.75~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.2.4
numpy            : 1.20.1
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 54.0.0
Cython           : None
pytest           : 6.2.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.2
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 3.0.6
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.2
sqlalchemy       : 1.4.1
tables           : None
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : None
numba            : None
```

</details>
