Latest mesa driver still shows black screen. I also tested with fresh Ubuntu 16.04 installation, and same thing happens there (black screen problems started well over a year ago, just it got worse this summer). AMDGPU-PRO 17.40 (and probably earlier ones) works and I don't have any problems with it. Switching to fullscreen and back is absolutely no problem with it, and it always shows picture at boot time in both monitors.