(In reply to Ludovico de Nittis from comment #0)
> When I try to boot SteamOS I end up with just a black screen.

The Xorg log file indicates that Xorg starts up in 2560x1440, then an X client
switches to 1920x1080 about 0.25s later (~23s after the kernel starts booting).
Do you notice anything on the monitor around that time, e.g. some kind of
flicker? What does the monitor say about the input signal when the black screen
is shown?

Does amdgpu.dc=0 on the kernel command line make any difference?

> Xorg.0.log contains this error but I'm not completely sure if it is the
> cause:
> (EE) AMDGPU(0): drmmode_do_crtc_dpms cannot get last vblank counter

It's probably not related to that.