iommu: tegra/gart: use correct gart_device

Pass the correct gart device pointer.

Change-Id: Ia54c3df7ce013855bf8843161f5ee0816482bda6
Reviewed-on: http://git-master/r/90064
Reviewed-by: Vandana Salve <vsalve@nvidia.com>
Tested-by: Vandana Salve <vsalve@nvidia.com>
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Hiroshi Doyu <hdoyu@nvidia.com>
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>