It should work with the FOSS amdgpu DDX (provided in a default installation by xf86-video-amdgpu), as long as its use isn't blocked. It's not uncommon for its use to be blocked by the presence of nomodeset on the kernel cmdline (in Grub), or by the installed presence of xf86-video-ati. What is output from:
What DDX is in use now? It's
very repeatedly announced in /var/log/Xorg.0.log. It's also reported by:
once
inxi is installed.