drm: rcar-du: Fix display timing controller parameter

There is a bug in the setting of the DES (Display Enable Signal)
register. This current setting occurs 1 dot left shift. The DES
register should be set minus one value about the specifying value
with H/W specification. This patch corrects it.

Signed-off-by: Koji Matsuoka <koji.matsuoka.xm@renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>