In a recent submission I was talking about AMD drivers in linux and mentioned how you'd need to get a custom kernel to be able to get your RX Vega to output video until the display code AMD has been working on hits upstream and that moment should be very close now.
Here is the initial pull request, and if the reviewer finds it good, it should be released along with kernel 4.15. If not, there some changes might be needed before the code is accepted, but at the very least the time is upon us.
The most important feature for Vega owners is that they won't need a custom kernel to get their GPU to display . But it'll also feature support for DP MST (display port), the base for freesync support and atomic mode setting.