Comment 6 for bug 1714881

Another user has report this on the nvidia linux board. This user reproducts the problem exactly, and reports a workaround which narrows down the problem. I haven't yet verified this workaround.
While this user has both intel and nvidia, he/she does not appear to be using a laptop.

Hey guys, I too have noticed something strange with GDM when trying to enable PRIME synchronization with modeset=1 for the nvidia_drm on Ubuntu 17.10.

I have a desktop with an Intel i7 Coffee Lake 8700K and Titan Xp and one monitor is plugged into the iGPU while the other is to the dGPU. When turning on KMS for the nvidia_drm during modprobe phase, GDM only starts on the iGPU monitor and no monitor on the dGPU is detected and a blank screen occurs.

This is fixed by kill GDM and initiating a sudo startx command from SSH which initiates a gnome-session with both monitors now detected and PRIME sync working on the iGPU monitor.

Whats' interesting is that immediately after killing the startx command, and restarting GDM, both monitors are now detected and logging in results in the iGPU monitor now showing PRIME sync in xrandr