drm/i915: CR clock gating is recommend to be set on PineView

The specs recommend that this bit be set on PineView. No reason is
given, but it sounds like a powersaving bit that we should expect the
BIOS to be setting...

v2: Rebase on top of _MASKED_ENABLE_BIT

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>