BUG: Fix bug with plot_white

[larsoner]

I use rank = compute_rank(...) regularly to process data. It works when then used as compute_covariance(..., rank=rank) and with make_inverse_operator(..., rank=rank). It should also work with plot_evoked_white.

EDIT: Note that the rank passed to make_inverse_operator etc. actually needs the "meg" key, not separate "mag" and "grad", so perhaps "meg" should actually be the preferred way to do it anyway. 🤷

Fixes failures seen in mne-tools/mne-bids-pipeline#1159

[larsoner]

@nordme can you review + merge if you're happy?

[larsoner]

CircleCI failure is the expected 45 missing refs, going to merge to keep things moving here and in mne-bids-pipeline!