drm: rcar-du: Convert rcar_du_encoders_init_one() return value to 0/<0

The function returns 1 on success, and either 0 or a negative error code
on failure. As the 0 and negative values don't need to be differentiated
by the caller, convert it to the usual scheme of returning 0 on success
and a negative error code on failure.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>