being not a radeon user myself, I guess you could file a bug report in http://bugs.dragonflybsd.org/ (you have to ask on IRC for an access) or come on the IRC channel (#dragonflybsd on EFNet).
From the latest release notes https://www.dragonflybsd.org/release56/, I read that :
- Major updates to the radeon and ttm (amd support code) drivers. We have not quite gotten the AMD support up to the more modern cards or Ryzen APUs yet, however.
Any way you should at least provide logs from the X server and from the system.