drm/i915: WaDisableVDSUnitClockGating isn't applicable to SNB

Can't find any mention of WaDisableVDSUnitClockGating ever being
relevant for SNB. Remove it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>