drm/i915: Eliminate weird parameter inversion from BXT PPS registers

v2: Keep using the same registers (PCH_*) instead of accidentally
    starting to use the other ones (BXT_*)2) (Jesse)

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>