How to reinstall default graphics drivers?

I'm running a custom Ubuntu 20.04 machine with an AMD Radeon RX 590 graphics card. A few days ago, I uninstalled the proprietary driver for my graphics card, but now I'm stuck at 1024x768 resolution and my computer tells me that the graphics card it's using is called "llvmpipe", which I assume is the onboard graphics card. It's still detecting the 590, and I'm plugged into it right now and getting graphical output fine, it's just not using it. A bootable USB confirmed that a fresh install doesn't have this problem, so something went wrong with my OS specifically. I'd very much prefer not to do a full reinstall, though, because I don't really have enough external storage to back up everything I have, so what packages contain the default graphics driver, and what do I need to do to make them start working again?

I should add that I've tried to reinstall the proprietary driver, but I get errors. Something to do with dpkg and amdgpu-dkms not configuring properly? Some Googling told me that it would be a problem with my kernel version, but downgrading to an older kernel didn't help, and the AMD helpline said that my kernel (5.4.0-58-generic) was supported. In any case, I'm not super concerned about whether I use the proprietary driver specifically or not. I'll be happy with any (minimal, software-based) solution that gets me back to 1080p.

EDIT: Running the amdgpu-install script with the option --no-dkms completes without errors but still doesn't fix my graphics on reboot. The make.log says something about "implicit declaration of function 'pci_platform_rom'.

EDIT 2: I sovled it! I used the instructions found here to completely purge my system of kernel 5.4.0-58 and replace it with 5.4.0-52, and I was able to install the amdgpu driver without incident.