Please run "lsmod | paste" and share the URL; let's see if there is more than one GPU detected. If there is and the BIOS doesn't provide options for disabling one of the cards we can blacklist modules to prevent them loading and confusing Xorg.
What is weird is that the hdmi output does output a signal, only without "content". So now in test setup it does 1080p/60 on my monitor. And in case of the television it also outputs 1080p.