Got it working for RX580 (Polaris), with kernel built out of re4drm56 branch, amdgpu option enabled in kernel and relevant polaris* firmware blobs copied to /libdata/firmware/amdgpu.
Boots without issues, with firmware initialized. After enabling the modesetting driver in /etc/xorg.conf, Xorg starts, but the screen is fully garbled, rendering it useless for the time being. Also getting identical error message on the VT console, as andreasdr - {drm:netbsd:amdgpu_cs_ioctl+0x434} ERROR Failed to initialize parser -87!
Probably more work on the driver required.