drm/plane: Add missing kerneldoc

The plane helpers aren't pulled into the DocBook yet, so these weren't
noticed.
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_plane_helper.h

@@ -49,6 +49,10 @@ extern int drm_crtc_init(struct drm_device *dev,
 
 /**
  * drm_plane_helper_funcs - helper operations for CRTCs
+ * @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
  *
  * The helper operations are called by the mid-layer CRTC helper.
  */