imx-drm: imx-drm-core: Export imx_drm_encoder_get_mux_id

imx-drm: imx-drm-core: Export imx_drm_encoder_get_mux_id

When building imx_v6_v7_defconfig with imx-drm drivers selected as modules, we
get the following build error:

ERROR: "imx_drm_encoder_get_mux_id" [drivers/staging/imx-drm/imx-ldb.ko] undefined!

Export the required function to avoid this problem.

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