drm/i915/cnl: Add max allowed Cannonlake DC.

This is a follow-up after enabling DC states with
commit: "drm/i915/DMC/CNL: Load DMC on CNL".

Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1499373908-26761-1-git-send-email-rodrigo.vivi@intel.com