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>