drm/i915: Return disconnected for SDVO DVI when there's no digital EDID.

The patch fixed a bug on MP965-D.  When VGA is connected to a DVI-I connector,
it incorrectly shows sdvo dvi as connected.

Signed-off-by: Ma Ling <ling.ma@intel.com>
[anholt: hand-resolved against previous commit and fixed up commit message]
Signed-off-by: Eric Anholt <eric@anholt.net>