drm: rcar-du: Use 1000 to avoid misunderstanding in rcar_du_dpll_divider()

It is difficult to understand its scale if number has many 0s.
This patch uses "* 1000" to avoid it in rcar_du_dpll_divider().

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>