iommu/amd: Remove double zero check

The free_pages() does zero check, therefore remove double zero
check here.

Signed-off-by: Libing Zhou <libing.zhou@nokia-sbell.com>
Link: https://lore.kernel.org/r/20200722064450.GA63618@hzling02.china.nsn-net.net


Signed-off-by: Joerg Roedel <jroedel@suse.de>