Having this same issue on kernel 4.13.5-1-ARCH.

CPU: AMD R7-1700X
Motherboard: AsRock Taichi X370
GPU: XFX RX-480 8GB

I have found a temporary solution that seems to allow me to actually boot and
use the card, however.

Pass the kernel parameter:
amdgpu.dpm=0
to shut off the Dynamic Power Management module, which contains AMD's
powerplay, and everything works fine.

I'm unsure what sort of limitations this introduces, but for the time being, it
will get your system working.