drm/tegra: Use IOMMU groups

In order to support IOMMUs more generically and transparently handle the
ARM SMMU on Tegra186, move to using groups instead of devices for domain
attachment. An IOMMU group is a set of devices that share the same IOMMU
domain and is therefore a good match to represent what Tegra DRM needs.

Signed-off-by: Thierry Reding <treding@nvidia.com>