I don't have a clue, what's going wrong. The only AMD system i have available here does not have any problem.
towo@Thinkpad:~$ inxi -SG
System: Host: Thinkpad Kernel: 4.17.1-towo.4-siduction-amd64 x86_64 bits: 64 Desktop: Xfce 4.12.4
Distro: siduction 18.2.0 Patience - xfce - (201803080000)
Graphics: Card-1: Advanced Micro Devices [AMD/ATI] Kaveri [Radeon R6/R7 Graphics] driver: radeon v: kernel
Card-2: Advanced Micro Devices [AMD/ATI] Jet PRO [Radeon R5 M230] driver: radeon v: kernel
Display: x11 server: X.Org 1.20.0 driver: ati,radeon unloaded: fbdev,modesetting,vesa
resolution: 1920x1080~60Hz
OpenGL: renderer: AMD KAVERI (DRM 2.50.0 4.17.1-towo.4-siduction-amd64 LLVM 6.0.0)
v: 4.5 Mesa 18.1.1
But that's radeon, not amdgpu. The latest kernel in our repo is equal to the aptosid kernel, which i have builded at least for testing. But since i do not have any polaris or vega hardware, especialy no Ryzen G, i can't test anything myself.