drm/i915/cnl: Add WaRsDisableCoarsePowerGating

Old Wa added now forever on CNL all steppings.

With CPU P states enabled along with RC6, dispatcher
hangs can happen.

Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222200535.9290-1-rodrigo.vivi@intel.com