iommu/tegra: gart: Fix NULL pointer dereference

Fix NULL pointer dereference on IOMMU domain destruction that happens
because clients list is being iterated unsafely and its elements are
getting deleted during the iteration.

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