iommu/amd: Fix double free of mem-region in error-path

When ioremap_nocache fails in iommu initialization the code
calls release_mem_region immediatly. But the function is
called again when the propagates into the upper init
functions leading to a double-free. Fix that.

Reported-by: Don Dutile <ddutile@redhat.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>