iommu/tegra: gart: Clean up driver probe errors handling

Properly clean up allocated resources on the drivers probe failure and
remove unneeded checks.

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