drm/mgag200: Provide per-device callbacks for BMC synchronization

Move the BMC-related code into its own file and wire it up with device
callbacks.

While programming a new display mode, G200EW3 and G200WB have to de-
synchronize with the BMC. Synchronization is done via VIDRST pins
and controlled via VRSTEN and HRSTEN bits. Move the BMC code behind
a serviceable interface and call it from the CRTC's enable and
disable functions.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728124103.30159-11-tzimmermann@suse.de