drm/sun4i: Fix a return value in case of error

If 'sun4i_backend_drm_format_to_layer()' does not return 0, then 'val' is
left unmodified.
As it is not initialized either, the return value can be anything.

It is likely that returning the error code was expected here.

As the only caller of 'sun4i_backend_update_layer_formats()' does not check
the return value, this fix is purely theorical.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>