I tried to purge xserver-xorg-video-amdgpu and xserver-xorg-video-ati. However it offered me to remove my entire desktop environment. So I hesitated. I had a Pop!_OS installed on my drive which also encountered the same graphical issues. So I instead installed a new Pop!_OS LTS on my drive. The new Pop!_OS also froze when the command `DRI_PRIME=1 glxgears -fullscreen` is passed. This should indicate that the graphical issue is not a result of my system tinkering. Since Pop!_OS is a derivative of Ubuntu of which is a derivative of Debian, I think the issue is either in Debian or upstream. I purged `firmware-amd-graphics` but the graphical issue still persists. I cannot seem to locate which package is causing the bug, so I am not sure how to file such an ambiguous bug.Try purging xserver-xorg-video-amdgpu and xserver-xorg-video-ati, then restarting X. That should automatically result in using the modesetting X driver. It does for my sea islands. If it's OK, either keep it that way, or try reinstalling only xserver-xorg-video-amdgpu.
Bests Regards, Guyenne