drm/tegra: Setup shared IOMMU domain after initialization

Move initialization of the shared IOMMU domain after the host1x device
has been initialized. At this point all the Tegra DRM clients have been
attached to the shared IOMMU domain.

This is important because Tegra186 and later use an ARM SMMU, for which
the driver defers setting up the geometry for a domain until a device is
attached to it. This is to ensure that the domain is properly set up for
a specific ARM SMMU instance, which is unknown at allocation time.

Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>