drm/fsl-dcu: use new drm_atomic_helper_shutdown

Commit 18dddadc ("drm/atomic: Introduce drm_atomic_helper_shutdown")
introduced a new helper to shutdown all CRTCs to replace the buggy
drm_crtc_force_disable_all() function. Make use of the new atomic
helper drm_atomic_helper_shutdown() to shutdown CRTCs.
Signed-off-by: Stefan Agner <stefan@agner.ch>

drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c

@@ -113,7 +113,7 @@ static void fsl_dcu_unload(struct drm_device *dev)
 {
 	struct fsl_dcu_drm_device *fsl_dev = dev->dev_private;
 
-	drm_crtc_force_disable_all(dev);
+	drm_atomic_helper_shutdown(dev);
 	drm_kms_helper_poll_fini(dev);
 
 	if (fsl_dev->fbdev)