drm/i915/gen9: fix watermarks when using the pipe scaler

Luckily, the necessary adjustments for when we're using the scaler are
exactly the same as the ones needed on ILK+, so just reuse the
function we already have.

v2: Invert the patch order so stable backports get easier.

Cc: stable@vger.kernel.org
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475872138-16194-1-git-send-email-paulo.r.zanoni@intel.com
(cherry picked from commit cfd7e3a20251b9ac95651d64556f87f86128a966)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>