iommu/tegra: gart: Simplify clients-tracking code

GART is a simple IOMMU provider that has single address space. There is
no need to setup global clients list and manage it for tracking of the
active domain, hence lot's of code could be safely removed and replaced
with a simpler alternative.

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