drm/ast: Add BMC virtual connector

Most aspeed devices have a BMC, which allows to remotely see the screen.
Also in the common use case, those servers don't have a display connected.
So add a Virtual connector, to reflect that even if no display is
connected, the framebuffer can still be seen remotely.
This prepares the work to implement a detect_ctx() for the Display port
connector.

v4: call drm_add_modes_noedid() with 4096x4096 (Thomas Zimmermann)
    remove useless struct field init to 0 (Thomas Zimmermann)
    don't use drm_simple_encoder_init() (Thomas Zimmermann)
    inline ast_bmc_connector_init() (Thomas Zimmermann)

Fixes: fae7d186 ("drm/probe-helper: Default to 640x480 if no EDID on DP")
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230713134316.332502-1-jfalempe@redhat.com