Hi Paolo,
did you download and install the amdgpu-pro package?
It is a bit of a “heart in your mouth” moment, because for me there is the fear of making my system unusable, and hard to return to its previous state.
However I have had success in Ubuntu and Manjaro with the -pro package. It is, unfortunately, not FOSS.
I’m thinking about trying the latest (update as at 20 March 2018) here because I do have some flicker issues that - once they start (sometimes after half an hour, sometimes never) - only a reboot solves.