AMD IOMMU: don't remove protection domain from iommu_pd_list

Impact: save unneeded logic to add and remove domains to the list

The removal of a protection domain from the iommu_pd_list is not
necessary. Another benefit is that we save complexity because we don't
have to readd it later when the device no longer uses the domain.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>