drm/mgag200: Remove disable handling from atomic_update

The primary plane has the atomic_disable helper set, so atomic_update
won't be called if the plane gets disabled. Remove the respective branch
from the helper.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230209154107.30680-4-tzimmermann@suse.de