drm: rcar-du: Output HSYNC instead of CSYNC

The DU outputs by default a composite sync signal (XOR of the horizontal
and vertical sync signals) on the HSYNC output pin. As this can confuse
devices and isn't needed by any of the supported encoders, configure the
HSYNC pin to output the horizontal sync signal.

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