TST/BUG: _gen_two_subplots always adding subplot even with passed axis
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

`kwargs.get("ax", fig.add_subplot(211))` was always adding a subplot (since the argument to `.get()` will be evaluated even if it doesn't need to be used). Assuming the intended behavior was for `fig.add_subplot(211)` to be called only if `ax` is not passed. This change also gets rid of half of remaining `matplotlib` warnings from 
`pandas/tests/plotting/test_hist_method.py::TestSeriesPlots::test_hist_legacy`
this a user facing change? if so pls add a whatsnew (bug fix plotting in 1.3)
can you see if we have any issues open that this would close  > this a user facing change? if so pls add a whatsnew (bug fix plotting in 1.3)
> can you see if we have any issues open that this would close

Don't think so, function is only used for testing plots (called from `_check_plot_works`) thanks @mzeitlin11 