drm: make crtc/encoder/connector/plane helper_private a const pointer

They're only used to store const pointers anyway. This helps to keep
Ville and the compiler happy.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/include/drm/drm_plane_helper.h b/include/drm/drm_plane_helper.h
index e48157a..96e1628 100644
--- a/include/drm/drm_plane_helper.h
+++ b/include/drm/drm_plane_helper.h
@@ -76,7 +76,7 @@
 static inline void drm_plane_helper_add(struct drm_plane *plane,
 					const struct drm_plane_helper_funcs *funcs)
 {
-	plane->helper_private = (void *)funcs;
+	plane->helper_private = funcs;
 }
 
 extern int drm_plane_helper_check_update(struct drm_plane *plane,