drm/i915/sdvo: Mark the status as unknown if attached with EDID

One problem with devices that share the DDC bus between the VGA and
DVI-I connectors is that with two devices attached we cannot know if
there is truly a monitor attached to the DVI connector. In this case, it
is preferrrable to mark the status as unknown, so that the user can
supply the known set of modes and continue to use the output.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>