drm/ast: Add an crtc_disable callback to the crtc helper funcs

Implement the proper CRTC disablement, just like done in mgag200
driver.

Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>