drm/plane: Make ->atomic_update() mandatory

There is no use-case where it would be useful for drivers not to
implement this function and the transitional plane helpers already
require drivers to provide an implementation.

v2: add new requirement to kerneldoc
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NThierry Reding <treding@nvidia.com>

drivers/gpu/drm/drm_atomic_helper.c

@@ -1108,7 +1108,7 @@ void drm_atomic_helper_commit_planes(struct drm_device *dev,
 
 		funcs = plane->helper_private;
 
-		if (!funcs || !funcs->atomic_update)
+		if (!funcs)
 			continue;
 
 		old_plane_state = old_state->plane_states[i];

include/drm/drm_plane_helper.h

@@ -52,7 +52,7 @@ extern int drm_crtc_init(struct drm_device *dev,
  * @prepare_fb: prepare a framebuffer for use by the plane
  * @cleanup_fb: cleanup a framebuffer when it's no longer used by the plane
  * @atomic_check: check that a given atomic state is valid and can be applied
- * @atomic_update: apply an atomic state to the plane
+ * @atomic_update: apply an atomic state to the plane (mandatory)
  *
  * The helper operations are called by the mid-layer CRTC helper.
  */