I think Linux was AMD Polaris support since 4.6, what seem to be a WIP on FreeBSD side. However the Roadmap seem slightly outdated since the x11-servers/wayland port already do exist, and this git wiki show they are indeed porting the features from 4.7.
I do not know what more in there could directly affect the support (or not) but the udev "HOLD ON" thing due to its future make me record Gentoo did a fork of it, called eudev, years ago to have a way to keep the option to maintain it (Gentoo) SystemD free, and it is already stable there...
I think I can maintain the WX 4100 in my future workstaion list.
Any idea if those updates could eventually come already to FreeBSD-11.1?