As far as I know [and I've checked very carefully}, the BIOS of the Dell XPS 8500 (gen 3) has no way to enable OR disable onboard video. It just works if correct cable connections are made..
When an add-in card is installed, onboard video is not inaccessible at all. If no -add-in video card is installed, there will be no video UNLESS a cable is attached from display to on-board HDMI or VGA in back, above audio ports.
You simply CAN'T use both an add-in card AND onboard video in Dell XPS 8500 (gen.3) at the same time. Since I don't have an earlier XPS 8500, I have no way of speaking for it.