drm/i915/cfl: Coffee Lake uses CNP PCH.

So let's force it on the virtual detection.

Also it is still the only silicon for now on this PCH,
so WARN otherwise.

v2: Rebased on top of Cannonlake and added the missed
    debug message as pointed by DK.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496937000-8450-2-git-send-email-rodrigo.vivi@intel.com