drm: rcar-du: error message is not needed for drm_vblank_init()

The only reason drm_vblank_init() could return an error at the
moment is a kcalloc() failure.
So we can remove current error message completely.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>