drm/i915/chv: Don't do group access reads from TX lanes either

Like PCS, TX group reads return 0xffffffff. So we need to target each
lane separately if we want to use RMW cycles to update the registers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>