Can't reproduce the cut-to-black issue, after several attempts (with either
'auto' or 'high' performance mode).

I did notice another issue while testing though, which is sort of related: If
leaving the GPU in 'high' performance mode while the display is turned off, it
still goes down to ~300 MHz core clock - except that it never goes back up to
high performance mode. This bug seems extremely similar to the one I had with
nvidia drivers (although nvidia drivers required an X restart, whereas simply
going from 'high' to 'auto' and back clears this issue).