drm/i915/cnl: WaThrottleEUPerfToAvoidTDBackPressure:cnl(pre-prod)

Wa for B-stepping only.

A for a hang issue that requires throttling EU performace
to 12.5% to avoid back pressure to thread dispatch

v2: Rebased. No change from v1.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Oscar Mateo <oscar.mateo@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170906220325.24524-1-rodrigo.vivi@intel.com