intel-iommu: Unlink domain from iommu

When we remove a device, we unlink the iommu from the domain, but
we never do the reverse unlinking of the domain from the iommu.
This means that we never clear iommu->domain_ids, eventually leading
to resource exhaustion if we repeatedly bind and unbind a device
to a driver.  Also free empty domains to avoid a resource leak.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Donald Dutile <ddutile@redhat.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@kernel.org