drm/i915: Change DCC tiling detection case to cover only mobile parts.

Later spec investigation has revealed that every 9xx mobile part has
had this register in this format.  Also, no non-mobile parts have been shown
to have this register.  So make all mobile use the same code, and all
non-mobile use the hack 965 detection.

Signed-off-by: Eric Anholt <eric@anholt.net>