I'm not sure where the kernel log is on arch. When I add that option to my
kernel command line that you recommended, both my drm-next-staging kernel and
4.16 mainline kernels fail. I have to remove it, then my 4.16 kernel works, but
the drm-next-staging kernel still fails to operate the screen. (The kernel
doesn't crash, as my keyboard still works, I can even press control-alt-delete
to reboot, so I suspect it just isn't using my screen in the amdgpu driver.)