drm/mgag200: Remove BMC output

Mgag200's BMC connector tracks the status of an underlying physical
connector and updates the BMC status accordingly. This functionality
works around GNOME's settings app, which cannot handle multiple
outputs on the same CRTC.

The workaround is now obsolete as the VGA-BMC connector handles BMC
support internally. Hence, remove the driver's code and the BMC output
entirely.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240805130622.63458-6-tzimmermann@suse.de