iommu/amd: Get rid of __unbind_pasid

Unbind_pasid is only called from mn_release which already
has the pasid_state. Use this to simplify the unbind_pasid
path and get rid of __unbind_pasid.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Oded Gabbay <Oded.Gabbay@amd.com>