PERF: fixup datetime_index repr perf (GH8556)
closes #8556 

low hanging fruit (and now is pretty scale independent).

```
In [1]: dr = pd.date_range('20000101', freq='D', periods=100000)

In [2]: %timeit dr._is_dates_only
10000000 loops, best of 3: 87.6 ns per loop

In [3]: pd.__version__
Out[3]: '0.15.0rc1-38-g857042f'
```

I do recall this slipping in sometime maybe in 0.14.0 was the changing in how the default display would be handled for dates with and w/o times.
