drm/i915: Reduce CHV DDL multiplier to 16/8

Apparently we must yet halve the DDL drain latency from what we're
using currently. This little nugget is not in any spec, but came
down through the grapevine.

This makes the displays a bit more stable. Not quite fully stable but at
least they don't fall over immediately on driver load.

v2: Update high_precision in valleyview_update_sprite_wm() too (Jesse)

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>