drm/mgag200: Add BMC output

The BMC output can be viewed via the BMC's web interface or a
similar client. Represent it as virtual encoder and connector.
It's attached to the same CRTC as the VGA connector.

The connector's status depends on the physical connector's status.
The BMC is only connected if the physical connector is not. This
is necessary to support userspace clients that can only handle a
single output per CRTC.

The BMC is a server feature. Add a BMC output for all server chips,
but not the desktop models.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610141141.29527-3-tzimmermann@suse.de