fix: error when passing axes to plot_compare_evokeds

[drammock]

Currently this:

mne.viz.plot_compare_evokeds(evoked, combine=None, picks=picks, axes=axs)

compares axes to a string, where axes is a list of matplotlib axes objects, and raises this:

ValueError                                Traceback (most recent call last)
<ipython-input-8-e36b566cce17> in <module>
----> 1 foo = mne.viz.plot_compare_evokeds({'mean': evoked}, combine=None, picks=picks, axes=axs)

/opt/mne/python/mne/viz/evoked.py in plot_compare_evokeds(evokeds, picks, colors, linestyles, styles, cmap, vlines, ci, truncate_yaxis, truncate_xaxis, ylim, invert_y, show_sensors, legend, split_legend, axes, title, show, combine, sphere)
   2252              .format(len(picks), combine))
   2253     # `combine` defaults to GFP unless picked a single channel or axes='topo'
-> 2254     if combine is None and len(picks) > 1 and axes != 'topo':
   2255         combine = 'gfp'
   2256     # convert `combine` into callable (if None or str)

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

This PR fixes it.

[larsoner]

LGTM +1 for merge

[agramfort]

thx @drammock