One solution would be to let EITHER
pci-stub (depends on whether you use the newer
-device vfio-pci or older
-device pci-assign) bind the card for passthrough first.
Depends on whether you load either of the modules and/or radeon in the initramfs, you might need to use a
softdep in /etc/modprobe.d to make sure
radeon is loaded later than the passthrough module. (Like if you load
vfio-pci in the initramfs but not
radeon, you don't need the softdep. If you load both
radeon in the initramfs OR both NOT in the initramfs, you probably need the softdep.
softdep radeon pre: vfio-pci
options vfio-pci ids=XXXX:XXXX
The "ids" can be checked by
lspci -nn. You may want to add the id of the HDMI Audio device of that card as well. The list is comma-separated (i.e. ids=XXXX:XXXX,YYYY:YYYY)
Btw, as long as you've done this correctly, you don't need to manually re-bind driver with sysfs/echo afterwards.