iommu/tegra: gart: Set aperture at domain initialization time

The aperture of the domain should always be available, otherwise drivers
need to attach first before they can use the aperture geometry.

Cc: Hiroshi Doyu <hdoyu@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>