drm/mgag200: vga-bmc: Transparently handle BMC

The VGA-BMC connector selects the VGA output if a display has been
attached to the physical connector. Otherwise it selects the BMC
output. In any case, the connector status is set to 'detected', so
that the userspace compositor displays to it.

Depending on the setting, the connector's display modes either come
from the VGA monitor's EDID or from an internal list of BMC-compatible
modes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240805130622.63458-3-tzimmermann@suse.de