drm/i915/skl+: Enable pipe CSC on cursor planes. (v2)

Extend this to SKL and BXT as it's needed for these platforms as well.

v2: Change if condition to HAS_DDI() instead of listing each platform
Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>