drm/i915/gen9: Cleanup skl_pipe_wm_active_state

This function is a wreck, let's help it get its life back together and
cleanup all of the copy pasta here.

Signed-off-by: Lyude <cpaul@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>