iommu/amd: Call memunmap in error path

Unmap old_devtb in error path.

Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Link: https://lore.kernel.org/r/20220301085626.87680-3-vasant.hegde@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>