drm/nouveau/kms/nv50-: use __drm_atomic_helper_plane_reset()

We have some of this open-coded already, use the helper to prevent problems
when adding (for example) support for the alpha property.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/dispnv50/wndw.c

@@ -538,9 +538,8 @@ nv50_wndw_reset(struct drm_plane *plane)
 
 	if (plane->state)
 		plane->funcs->atomic_destroy_state(plane, plane->state);
-	plane->state = &asyw->state;
-	plane->state->plane = plane;
-	plane->state->rotation = DRM_MODE_ROTATE_0;
+
+	__drm_atomic_helper_plane_reset(plane, &asyw->state);
 }
 
 static void