staging: imx_drm: imx_ldb: fix 18 bit format

Since 18 bit is supported as datawidth in device tree it should be
supported in driver. Beside the LDB channel the IPU channel has also
to be configured to use BGR666.

Signed-off-by: Markus Niebel <Markus.Niebel@tqs.de>
Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>