drm/i915: add ValleyView clock gating init

Set required clock gating and chicken bits on VLV.

v2: set PIXEL_SUBSPAN_COLLECT_OPT_DISABLE too (Ben)
    move function below ivb version to pretend to be consistent (Ben)

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>