drm/i915/cnl: Remove unnecessary check in cnl_setup_private_ppat

There is no need check if PPGTT is disabled because that not possible
in CNL. Execlists and GuC submission modes rely on at least aliasing
PPGTT and even intel_sanitize_enable_ppgtt says: "We don't allow disabling
PPGTT for gen9+ as it's a requirement for execlists, the sole mechanism
available to submit work."

Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171027223207.7869-1-michel.thierry@intel.com