drm/mgag200: Implement struct drm_plane_helper_funcs.atomic_enable

Enable the primary plane for mgag200 hardware via atomic_enable.
Atomic helpers invoke this callback only when the plane becomes
active.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230209154107.30680-5-tzimmermann@suse.de