drm: arm: malidp: Use drm_atomic_helper_shutdown() to disable planes on removal

The plane cleanup handler currently calls drm_plane_helper_disable(),
which is a legacy helper function. Replace it with a call to
drm_atomic_helper_shutdown() at removal time.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>