drm/helper: lock all around force mode restore

Since Daniel documented things with a sledge hammer, we got lots of
nice backtraces in suspend/resume operations, I've check the callers
of this and they all seems safe to me,

This fixes one set of warns I reported.
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc_helper.c

@@ -983,6 +983,7 @@ void drm_helper_resume_force_mode(struct drm_device *dev)
 	int encoder_dpms;
 	bool ret;
 
+	drm_modeset_lock_all(dev);
 	list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
 
 		if (!crtc->enabled)
@@ -1017,6 +1018,7 @@ void drm_helper_resume_force_mode(struct drm_device *dev)
 
 	/* disable the unused connectors while restoring the modesetting */
 	__drm_helper_disable_unused_functions(dev);
+	drm_modeset_unlock_all(dev);
 }
 EXPORT_SYMBOL(drm_helper_resume_force_mode);