drm: drm_plane_force_disable is not for atomic drivers

This way I can explain why it'll be fine to pass a NULL acquire ctx
here in the next patch.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322215058.8671-4-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_plane.c

@@ -277,6 +277,12 @@ EXPORT_SYMBOL(drm_plane_from_index);
  *
  * Used when the plane's current framebuffer is destroyed,
  * and when restoring fbdev mode.
+ *
+ * Note that this function is not suitable for atomic drivers, since it doesn't
+ * wire through the lock acquisition context properly and hence can't handle
+ * retries or driver private locks. You probably want to use
+ * drm_atomic_helper_disable_plane() or
+ * drm_atomic_helper_disable_planes_on_crtc() instead.
  */
 void drm_plane_force_disable(struct drm_plane *plane)
 {
@@ -285,6 +291,8 @@ void drm_plane_force_disable(struct drm_plane *plane)
 	if (!plane->fb)
 		return;
 
+	WARN_ON(drm_drv_uses_atomic_modeset(plane->dev));
+
 	plane->old_fb = plane->fb;
 	ret = plane->funcs->disable_plane(plane);
 	if (ret) {