I've blogged about getting Manjaro Linux to work with my AMD RX 470 before. The method described in that post got my AMD RX 470 graphics card working with the default 4.4 kernel. This worked fine - with the usual caveats regarding VESA software rendering - until I tried to upgrade to newer versions of the kernel.
My understanding is that the 4.4 kernel series doesn't include drivers for the relatively recent AMD RX 470 GPUs, whereas later kernel series (4.8 and 4.9 specifically) do. Unfortunately trying to boot into a 4.9 kernel resulted in the X server locking up so well that even the usual Alt-Fx didn't get me to a console to fix the problem.
First, while still on the 4.4 kernel, enable sshd to be able to remote into the system. That's probably a good idea anyway in case you bork the GUI in new and interesting ways. This is how you enable sshd and start it on Manjaro:
At this point, reboot into the newer kernel and ssh into the machine, then use mhwd to remove the VESA video driver:
Reboot from ssh and the system came up with X running the Gallium driver:
The faffing about with enabling sshd and booting into the 4.9 kernel might not be strictly necessary. Given that I wasn't able to get 4.4 working with video-amdgpu, this approach looked like a safer route compared to trying to remove video-vesa while still running the 4.4 kernel.
Anyway, it's nice to be able to run X with actual hardware rendering and acceleration.