Here's a bit of background which may help.
- The driver is named "radeondrm" and the driver's first (and only) instance is #0, so your kernel messages refer to device "radeondrm0".
- Usually, when there may be multiple instances of a driver, the kernel configuration uses object-oriented class definitions to allow the kernel to instantiate each instance, without specific defined numbering. The rules are defined in the files.conf(5) man page, and you can see the rule for radeondrm(8) in the man page SYNOPSIS section, where it is listed as `radeondrm* at pci?`.
- Lastly, the disable command you are placing into the /etc/bsd.re-config file is described in the config(8) man page.
Once you have the configuration change in the /etc/bsd.re-config file, you need to relink a new kernel with the revised configuration. Run /usr/libexec/reorder_kernel to do that, then reboot.