Thanks, seems indeed the legacy driver is no longer supported, and even without HDMI plugged in and nothing graphical being installed, there's something using the graphics hardware in the background for some reason. I'm not using the graphics drivers myself, just the mailbox, and occasionally write to the framebuffer from the CPU for debugging.
Anyway, seems I'll have to stick to 16.x indefinitely, it's been super stable anyway, and for an embedded device, that's all I need. Thanks!