We have some long-standing bug reports about this. Currently the driver
will panic if multiple agp devices are present. The prevailaing
workaround is simply to disable all but one using device hints, so let's
modify the driver to do that automatically rather than panicking.
This does not go as far as the patch in PR 187015, which assigns unit
numbers to all but the first device. I am not sure if that approach is
really useful and don't have any hardware to try it, but if so I can
modify the patch.