drm/i915: Reorganize VLV DDL setup

Introduce struct vlv_wm_values to house VLV watermark/drain latency
values. We start by using it when computing the drain latency values.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>