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 --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 5de6cf4..b00d323 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -6018,6 +6018,7 @@
 #define GEN6_RSTCTL				0x9420
 
 #define GEN8_UCGCTL6				0x9430
+#define   GEN8_GAPSUNIT_CLOCK_GATE_DISABLE	(1<<24)
 #define   GEN8_SDEUNIT_CLOCK_GATE_DISABLE	(1<<14)
 
 #define GEN6_GFXPAUSE				0xA000