drm/doc: introduce new section for standard plane properties

Introduce a new "Standard Plane Properties" section for properties
defined in drm_plane.c. Move the mis-placed IN_FORMATS docs there.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201217113220.102271-3-contact@emersion.fr