drm: rcar-du: Fix LVDS start sequence on Gen3

According to the latest revision of the datasheet, the LVDS I/O pins
must be enabled before starting the PLL. Fix it.

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