drm/i915/skl: Implement WaSetGAPSunitClckGateDisable

Let's also take the opportunity the remove the comment telling it's a
pre-prod W/A, it should be obvious from the stepping test.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>