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