gpu: host1x: Use IOMMU groups

Use IOMMU groups to attach the host1x device to its IOMMU domain. This
is not strictly necessary because the domain isn't shared with any other
device, but it makes the code consistent with how IOMMU is handled in
other drivers and provides an easy way to detect when no IOMMU has been
attached via device tree.

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