drm/sun4i: Propagate error to the caller

If 'sun4i_layers_init()' returns an error, propagate it instead of
returning -EINVAL unconditionally.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>