drm/i915/skl: Remove WaDisableVFUnitClockGating

Dropping it because it is for pre-production stepping, also removed
bit definition in i915_reg.h as it is not used anywhere else.

Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Keep #define as Ville suggested.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>