iommu/tegra-gart: Make use of domain_alloc and domain_free

Implement domain_alloc and domain_free iommu-ops as a
replacement for domain_init/domain_destroy.

Tested-by: Thierry Reding <treding@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>