drm: Add support for optional per-plane rotation property

Not all planes on the system may support the same rotations/reflections,
so make it possible to create a separate property for each plane.
This way userspace gets told exactly which rotations/reflections are
possible for each plane.

v2: Add drm_plane_create_rotation_property() helper
v3: Drop the BIT(), __builtin_ffs(x) - 1,
    Moar WARNs for bad parameters
    Deal with superfluous code shuffling

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1474907460-10717-4-git-send-email-ville.syrjala@linux.intel.com

include/drm/drm_plane.h

@@ -385,6 +385,7 @@ enum drm_plane_type {
  * @type: type of plane (overlay, primary, cursor)
  * @state: current atomic state for this plane
  * @zpos_property: zpos property for this plane
+ * @rotation_property: rotation property for this plane
  * @helper_private: mid-layer private data
  */
 struct drm_plane {
@@ -431,6 +432,7 @@ struct drm_plane {
 	struct drm_plane_state *state;
 
 	struct drm_property *zpos_property;
+	struct drm_property *rotation_property;
 };
 
 #define obj_to_plane(x) container_of(x, struct drm_plane, base)