drm/i915/mst: Document the userspace fail with possible_crtcs

drm/i915/mst: Document the userspace fail with possible_crtcs

To avoid accidentally breaking things in the future add a
comment explaining why we misconfigure the pipe_mask.

Also toss in a TODO for investigating a single encoder
approach as opposed to the encoder-per-pipe approach.

v2: Drop a bogus TODO comment

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002162505.30716-6-ville.syrjala@linux.intel.com
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>