iommu: virtio: Use generic_iommu_put_resv_regions()

Use the new standard function instead of open-coding it.

Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: virtualization@lists.linux-foundation.org
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>