drm/tegra: hdmi: Register debugfs in ->late_register()

The ->late_register() and ->early_unregister() callbacks are called at
the right time to make sure userspace only accesses interfaces when it
should. Move debugfs registration and unregistration to these callback
functions to avoid potential races with userspace.
Signed-off-by: NThierry Reding <treding@nvidia.com>