drm/i915/skl: Remove linetime from skl_wm_values

Next part of cleaning up the watermark code for skl. This is easy, since
it seems that we never actually needed to keep track of the linetime in
the skl_wm_values struct anyway.

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