drm: Remove some unused stuff from drm_plane

There's a bunch of unused members inside drm_plane, bloating the size of
the structure needlessly. Eliminate them.

v2: Remove all of it from kernel-doc too

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>