drm/i915: Fix the missing PPAT cache attributes on CNL

Add back the GEN8_PPAT_WB cache attributes in cnl_setup_private_ppat(),
which are missed on CNL.

Fixes: 4e34935f ("drm/i915/cnl: Setup PAT Index.")
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1504208177-27784-1-git-send-email-zhi.a.wang@intel.com