The "[drm:amdgpu_init [amdgpu]] *ERROR* VGACON disables amdgpu kernel modesetting." in your dmesg output comes from the 'nomodeset' parameter. If possible, I'd try setting up an SSH daemon, removing 'nomodeset' from your kernel command-line, and getting dmesg output remotely. (I assume you don't get any screen output without 'nomodeset'.)
I had the same issue with my RX460. Modesetting (to my knowledge) isn't possible without the nonfree firmware, so I just gave in and created a package for the blobs. There _may_ be a way to use your card without the firmware, but if there is, I'm not aware of it. Sorry :< Jakob (Apologies if you're receiving this twice, Gnus quit out on me as I was sending this).
