drm/ast: Disconnect BMC if physical connector is connected

Many user-space compositors fail with mode setting if a CRTC has
more than one connected connector. This is the case with the BMC
on Aspeed systems. Work around this problem by setting the BMC's
connector status to disconnected when the physical connector has
a display attached. This way compositors will only see one connected
connector at a time; either the physical one or the BMC.

Suggested-by: Jocelyn Falempe <jfalempe@redhat.com>
Fixes: e329cb53 ("drm/ast: Add BMC virtual connector")
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: <stable@vger.kernel.org> # v6.6+
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116130217.22931-1-tzimmermann@suse.de