drm/i915: Be extra careful about A/D matching for multifunction SDVO

If we're both RGB and TMDS capable, we'll have set up one connector for
each.  When determining connectivity, require analog/digital state in
the EDID block to match analog/digital support in the connector.
Otherwise, both DVI and VGA will appear to be connected.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>