drm/i915: Always use SDVO_B detect bit for SDVO output detection.

drm/i915: Always use SDVO_B detect bit for SDVO output detection.

After the following commit is shipped, the SDVO C detection will depend on
the SDVO_C/DP detion bit.
commit 13520b051e8888dd3af9bda639d83e7df76613d1
Author: Kristian Høgsberg <krh@redhat.com>
Date:   Fri Mar 13 15:42:14 2009 -0400

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

According to the spec we should continue to detect the SDVO_B/C based on
the SDVO_B detection bit.  The new detection bit on G4X platform is for
the HDMI_C detection rather than SDVO_C detection.

https://bugs.freedesktop.org/show_bug.cgi?id=20639

Signed-off-by: Ma Ling <ling.ma@intel.com>
Acked-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>