Move the native-mode property inside the display-timings node.

According to
Documentation/devicetree/bindings/display/panel/display-timing.txt.
native-mode is a property of the display-timings node.

If it's located outside of display-timings, the native-mode setting is
ignored and the first display timing is used (which is a problem only if
someone adds another display timing).
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>