Oh, you have Polaris. Me too.
I have had issues getting it to run. I think when you boot in UEFI mode the driver has a conflict with the console and prevents starting graphics. The workaround is to set
hw.syscons.disable=1
in
/boot/loader.conf. See if you have this set at your side.
Here is the thread where I shared my experience (although the info is a bit older, so keep this in mind).
I have FreeBSD 11.1 p10 running kde4 desktop on a ryzen 7 1700. I was using a hd 7970 for my video card, and everything worked fine, i upgraded to a rx580 and now x fails to start. I could not find anything that said it is a supported card. I did not want to "-configure" because of "Video cards...
forums.freebsd.org