drm/i915: Use ilk_compute_wm_level to compute WM_PIPE values

Unify the code a bit to use ilk_compute_wm_level for all watermark
levels.

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