drm/i915: implement WaDisableVLVClockGating_VBIIssue on VLV

This allows us to get the right vblank interrupt frequency.

v2: pull in register definition

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Antti Koskipää <antti.koskipaa@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>