drm/imx: imx-ldb: allow to determine bus format from the connected panel

This patch makes the fsl,data-width and fsl,data-mapping device tree
properties optional if a panel is connected to the ldb output port
via the of_graph bindings. The data mapping is determined from the
display_info.bus_format field provided by the panel.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>