staging: imx-drm: imx-ldb: Staticize of_get_data_mapping()

Staticize of_get_data_mapping() in order to fix the following sparse warning:

drivers/staging/imx-drm/imx-ldb.c:424:11: warning: symbol 'of_get_data_mapping' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>