imx-drm: imx-ldb: Check 'mode' pointer first

Fix the following static checker warning:

drivers/staging/imx-drm/imx-ldb.c:109 imx_ldb_connector_get_modes() error: potential null dereference 'mode'.  (drm_mode_create returns null)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>