hdmi_ignore_hotplug=1 turns it off. Unfortunately since the astroberry VNC setup is sharing the X11/HDMI desktop, VNC stops working after you do this and just gives a black screen with the message: "can not currently show desktop". On my other setup I ran a VNC separate from the X11 desktop and was able to shut off HDMI output. That's why that setup worked with WIFI and USB3.
OH, and I was mistaken about the distro that I used for my previous setup. It was actually raspbian, not mate.