drm/rcar-du: Write DPTSR only if the second source exists

drm/rcar-du: Write DPTSR only if the second source exists

Currently the driver always writes DPTSR when setting up the hardware.
However, writing the register is only meaningful when the second source
for a plane is used, and the register is not even documented for SoCs
that do not have the second source.

So move the write behind a condition.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com> # On R-Car M3-N
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241217-rcar-gh-dsi-v5-2-e77421093c05@ideasonboard.com