drm/mgag200: Add per-device callbacks

While currently empty, the device callbacks will allow mgag200's
modesetting code to interact with the BMC and PIXPLLC.

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/20220728124103.30159-10-tzimmermann@suse.de