drm/tegra: Store parent pointer in Tegra DRM clients

Tegra DRM clients need access to their parent, so store a pointer to it
upon registration. It's technically possible to get at this by going via
the host1x client's parent and getting the driver data, but that's quite
complicated and not very transparent. It's much more straightforward and
natural to let the children know about their parent.

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