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>

diff --cc drivers/gpu/drm/i915/i915_reg.h
Simple merge

diff --cc drivers/gpu/drm/i915/intel_pm.c
Simple merge