drm/vc4: Use drm_atomic_helper_shutdown

drm/vc4: Use drm_atomic_helper_shutdown

drm_plane_helper_disable is a non-atomic drivers only function, and
will blow up (since no one passes the locking context it needs).

Atomic drivers which want to quiescent their hw on unload should
use drm_atomic_helper_shutdown() instead.

v2: Rebase.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-14-daniel.vetter@ffwll.ch