drm: add docs for standard CRTC properties

This patch adds docs for the ACTIVE and MODE_ID CRTC properties.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/k52vYFBQ5ZO18TgZl3W8MgP6f4qu5Ncir7w-On8Dm0V2KTAcVkUoS7-IGPcvDJAXLsyJAUsD0QFJts3Dy0yWyHXVh85axrZkybh3MGGFhQc=@emersion.fr