drm/i915: Clean up WRPLL/SPLL #defines

Luckily the bit definitions match, but it's still confusing
to use one when handling the other. So sprinkle some OCD over
the #defines to make them match and use the right version in
each place.

Maybe we should unify these definitions completely, but that
can always be done sometime in the future.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>