drm/of: free the iterator object on failure

When bailing out due to the sanity check the iterator value needs to be
freed because the early return prevents for_each_child_of_node() from
doing the dereference itself.

Fixes: 65290075 ("drm: of: Add drm_of_lvds_get_dual_link_pixel_order")
Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714143300.20632-1-steven.price@arm.com