Debian 10.6 boots to black screen; "no screens found(EE) in log"

I have installed Debian 10.6 "Buster" on a 2019 5k iMac, but the system boots to a black screen with a flashing cursor in the upper-left corner. I can still get to a console, however. In looking at /var/log/Xorg.o.log, I see the following errors: (EE) Screen 0 deleted because of no matching config section (II) UnloadModule: "amdgpu" (EE) Screen 0 deleted because of no matching config section (II) UnloadModule: "amdgpu" (EE) Device(s) detected, but none match those in the config file. (EE) Fatal server error: no screens found(EE) I generated an xorg.conf file by running X -configure and copied it to /etc/X11/xorg.conf, but got the same results. The only difference is that before, Xorg.0.log contained a long list of informational messages about the various supported GPUs with the AMDGPU driver, whereas now it just says All GPUs supported by the amdgpu kernel driver. I'm pretty sure this is a problem with the screen, not the driver, because booting with an Ubuntu Live DVD works fine (I presume it uses the same amdgpu driver). But I don't really know how to modify Xorg's config, change the driver, or change the screen config. Typing dpkg-reconfigure -Phigh xserver-xorg does literally nothing. I've been searching on this issue for a while, but all I see are posts from literally 10-15 years ago. Most of the information in those posts are irrelevant now, and I don't know enough about the low-level nuts and bolts of X configuration to adapt any of it to modern times. How do I fix this?