drm/i915: Add rotation property for sprites

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>