How to uninstall AMDGPU?

Similar happened to me trying to figure out settings and drivers for Mythtv on 18.04 with Kaveri. If 'sudo amdgpu-pro-uninstall' didn't return errors, and the dpkg --purge didn't work (I also did both). Then my many driver install/uninstall resulted in missing files for the radeon driver fixed for me with

sudo apt-get install --reinstall xserver-xorg-video-radeon

And if that fails, then maybe uncomment Enable=true under debug in /etc/gdm3/custom.conf. Then use Virtual Terminals (I ssh in from my laptop) to switch between multi-user and graphical.target so you can review ~/.local/share/xorg/Xorg.0.conf for more clues. Similar debugging and searching on askubuntu led me to the reinstall above.

I got also the same problem, and those commands helped me:

sudo apt-get install --reinstall xserver-xorg-video-radeon xserver-xorg-core
sudo apt install xserver-xorg
sudo update-initramfs -u -k all