I cant seem to get this to work as after i install the drm-kmod and add kld_list="amdgpu" to rc.conf after the freebsd boot screen the monitor go blank. turning off the frame buffer doesnt help.
This wiki has some additional steps that are not listed in your post.
https://wiki.freebsd.org/Graphics#AMD_Graphics
when I i read dmesg without the driver loaded it recogninzes it as a ryzen 3 cpu but once i load the driver the screen goes blank and I cant read the dmesg.I'm running the same APU in OpenBSD current.
http://daemonforums.org/showpost.php?p=67112&postcount=125
My understanding is that the firmware is bundled with the module. I would review your dmesg to see if it recognized and the firmware loaded. Also /var/log/Xorg.0.log for other errors
In OpenBSD I had to tweak the mouse cursor to a software cursor but otherwise, it runs well.