drm/ast: vga: Transparently handle BMC support

Permanently set the connector status to 'connected'. Return BMC modes
for connector if no display is attached to the physical DP connector.
Otherwise use EDID modes as before.

If the status of the physical connector changes, the driver still
generates a hotplug event. DRM clients will then reconfigure their
output to a mode appropriate for either physical display or BMC.

v3:
- use struct ast_connector.physical_status to handle BMC

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240815151953.184679-11-tzimmermann@suse.de