iommu/tegra: gart: Fix spinlock recursion

Fix spinlock recursion bug that happens on IOMMU domain destruction if
any of the allocated domains have devices attached to them.

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