iommu/tegra: gart: Allow only one active domain at a time

GART has a single address space that is shared by all devices, hence only
one domain could be active at a time.

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