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>

drivers/staging/imx-drm/imx-drm-core.c

@@ -678,6 +678,7 @@ int imx_drm_encoder_get_mux_id(struct imx_drm_encoder *imx_drm_encoder,
 
 	return i;
 }
+EXPORT_SYMBOL_GPL(imx_drm_encoder_get_mux_id);
 
 /*
  * imx_drm_remove_encoder - remove an encoder