video: of: display_timing: fix default native-mode setting

Set native mode to the first child node of the display-timings node and not
the first child node of the display-timings parent node.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>