drm/doc: document drm_mode_get_plane

Add a small description and document struct fields of
drm_mode_get_plane.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611213516.77904-2-leandro.ribeiro@collabora.com