BUG: hidden ticklabels with sharex and secondary
Closes https://github.com/pydata/pandas/issues/9158

``` python
import pandas as pd
d = {'A' : [1., 2., 3., 4.], 'B' : [4., 3., 2., 1.], 'C': [5, 1, 3, 4]}
df = pd.DataFrame(d, index=pd.date_range('2014 10 11', '2014 10 14'))

axes = df[['A','B']].plot(subplots=True)
df['C'].plot(ax=axes[0], secondary_y=True)   # x tick labels dissappear when this line is executed
```

![gh](https://cloud.githubusercontent.com/assets/1312546/7216419/ffe3bc2e-e5c4-11e4-89de-d18e5e59d3b0.png)

Introduced in the subplot refactoring in https://github.com/pydata/pandas/pull/7457

cc @sinhrks

The fix is pretty simple, but I'm worried I may have broken other things. No tests failed though, so hopefully we're good.
