drm/plane: add inline doc for struct drm_plane

Some of the members of struct drm_plane had extra comments so for these
add inline kernel comment to consolidate all documentation in one place.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
[danvet: Bikeshed a bit more to have real paragraphs with real
sentences.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1478513013-3221-4-git-send-email-gustavo@padovan.org