drm/i915: Simplify DP port limited color range bit platform checks

Instead of checking for everything not supporting the limited color
range bit in the DP port register, let's check for the one thing
that does have it (g4x).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479145447-12907-3-git-send-email-ville.syrjala@linux.intel.com