drm/vc4: debugfs: Return an error on failure

vc4_debugfs_add_file() can fail, so let's propagate its error code instead
of silencing it.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220711173939.1132294-64-maxime@cerno.tech