drm/i915/chv: Don't use PCS group access reads

All PCS groups access reads return 0xffffffff, so we can't use group
access for RMW cycles. Instead target each spline separately.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
[danvet: Fight conflict with misplaced ; .... ARGH!]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>