drm/imx: imx-ldb: honor 'native-mode' property when selecting video mode from DT

This patch allows to select a specific video mode from a list of modes
defined in DT by setting the 'native-mode' property appropriately.

This change does not affect the behaviour of existing platforms, since
they either:
   - have just one display-timings subnode
   - have the native-mode property pointing to the first entry
   - let the bootloader select the appropriate timing

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>