drm/i915/sdvo: Always fallback to querying the shared DDC line

On a few devices, like the Mac Mini, the CRT DDC pins are shared between
the analog connector and the digital connector. In this scenario, rely
on the EDID to determine if a digital panel is connected to the digital
connector.

Reported-and-tested-by: Tino Keitel <tino.keitel@tikei.de>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>