drm/i915/sdvo: Restore guess of the DDC bus in absence of VBIOS

If the VBIOS tells us the mapping of the SDVO device onto the DDC bus,
use it. However, if there is no VBIOS available that mapping is
uninitialised and we should fallback to our earlier guess.

Fix regression introduced in b1083333 (which in turn is a fix for the
regression caused by the introduction of this guess, 14571b4c).

References:

  Bug 29499 - [945GM] Screen disconnected because of missing VBIOS
  https://bugs.freedesktop.org/show_bug.cgi?id=29499

  Bug 15109 - i945GM fails to detect EDID on DVI port
  https://bugzilla.kernel.org/show_bug.cgi?id=15109



Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: Paul Neumann <paul104x@yahoo.de>
Cc: Adam Jackson <ajax@redhat.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: stable@kernel.org