drm/i915/cnl: Cannonlake uses CNP PCH.

Avoid warning when CNP is detected with CNL.

Also let's force it on the virtual detection.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496781040-20888-2-git-send-email-rodrigo.vivi@intel.com