drm/i915/cnl: document WaVFUnitClockGatingDisable

No functional change. WA is already properly applied.
but in different databases it has different names.
Let's document all of them to avoid future confusion.

Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180306012000.18928-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -8523,6 +8523,7 @@ static void cnl_init_clock_gating(struct drm_i915_private *dev_priv)
 	I915_WRITE(SLICE_UNIT_LEVEL_CLKGATE, val);
 
 	/* WaDisableVFclkgate:cnl */
+	/* WaVFUnitClockGatingDisable:cnl */
 	val = I915_READ(UNSLICE_UNIT_LEVEL_CLKGATE);
 	val |= VFUNIT_CLKGATE_DIS;
 	I915_WRITE(UNSLICE_UNIT_LEVEL_CLKGATE, val);