iommu: tegra/gart: use correct gart_device

Pass the correct gart device pointer.

Reviewed-by: Vandana Salve <vsalve@nvidia.com>
Tested-by: Vandana Salve <vsalve@nvidia.com>
Reviewed-by: Hiroshi Doyu <hdoyu@nvidia.com>
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>
Signed-off-by: Hiroshi DOYU <hdoyu@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>