drm: arm: hdlcd: 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. The plane .destroy()
handler now consisting only of a call to drm_plane_cleanup(), replace it
with direct calls to that function.

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