drm/vc4: debugfs: Simplify debugfs registration

The vc4 has a custom API to allow components to register a debugfs file
before the DRM driver has been registered and the debugfs_init hook has
been called.

However, the .late_register hook allows to have the debugfs file creation
deferred after that time already.

Let's remove our custom code to only register later our debugfs entries as
part of either debugfs_init or after it.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>