iommu/amd: Handle race between registration and device probe

As for the Intel driver, make sure the AMD driver can cope with seeing
.probe_device calls without having to wait for all known instances to
register first.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/a8d8ebe12b411d28972f1ab928c6db92e8913cf5.1660572783.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>