drm/mgag200: Remove vidrst callbacks from struct mgag200_device_funcs

The callbacks disable_vidrst and enable_vidrst are obsolete. Remove
the fields from struct mgag200_device_funcs. Instead call their
implementations directly of the field 'has_vidrst' has been set in
struct mgag200_device_info.

Also change the logic slightly. The BMC used to start and stop scanout
during the CRTC's atomic_enable and atomic_disable. Plane updates were
done while the BMC scanned out the display. Now only stop once in
atomic_disable at the beginning of a modeset and then restart the
scanout at the end of a modeset in atomic_enable. While the modeset
takes place, the BMC does not scanout at all.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240711072415.11831-3-tzimmermann@suse.de