Sprite planes support 180 degree rotation. The lower layers are now in
place, so hook in the standard rotation property to expose the feature
to the users.

v2: Moving rotation_property to mode_config

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>