drm/ast: Remove BMC output

Ast'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 all code for physical outputs
handle BMC support internally. Hence, remove the driver's code and
the BMC output entirely.

v3:
- remove struct ast_bmc_connector

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240815151953.184679-12-tzimmermann@suse.de