drm/i915: Read the right SDVO register when detecting SVDO/HDMI.

This fixes incorrect detection of the second SDVO/HDMI output on G4X, and
extra boot time on pre-G4X.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>