drm/i915/cnl: Enable fifo underrun for Cannonlake.

Also in a way that reuse bdw+ for all next platforms.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497047175-27250-16-git-send-email-rodrigo.vivi@intel.com