drm/ast: report connection status on Display Port.

Aspeed always report the display port as "connected", because it
doesn't set a .detect_ctx callback.
Fix this by providing the proper detect callback for astdp and dp501.

This also fixes the following regression:
Since commit fae7d186 ("drm/probe-helper: Default to 640x480 if no
EDID on DP") The default resolution is now 640x480 when no monitor is
connected. But Aspeed graphics is mostly used in servers, where no monitor
is attached. This also affects the remote BMC resolution to 640x480, which
is inconvenient, and breaks the anaconda installer.

v2: Add .detect callback to the dp/dp501 connector (Jani Nikula)
v3: Use .detect_ctx callback, and refactors (Thomas Zimmermann)
    Add a BMC virtual connector
v4: Better indent detect_ctx() functions (Thomas Zimmermann)
v5: Enable polling of the dp and dp501 connector status
    (Thomas Zimmermann)
v6: Change check order in ast_astdp_is_connected (Jammy Huang)

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/20230713134316.332502-2-jfalempe@redhat.com