Comment # 23 on bug 103234 from Marek Olšák

The xorg ati driver is for the radeon kernel module.
The xorg amdgpu driver is for the amdgpu kernel module.

If the xorg driver is missing for the respective kernel module, the X server
will use the generic xorg modesetting driver instead. The behavior might be the
same because the modesetting driver has similar capabilities as the hw-specific
drivers.