compositor-drm: Comment struct members

Clarify the difference between crtc_id (DRM object) and pipe (index into
drmModeRes->crtcs array, possible_crtcs bitmask).

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
Differential Revision: https://phabricator.freedesktop.org/D1405