drm/mgag200: Add VGA-BMC output

Duplicate VGA output to VGA-BMC output and update all code for Matrox
server chips. The new output represents a VGA output that has a BMC
attached to it. No functional changes so far.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240805130622.63458-2-tzimmermann@suse.de