drm/imx: imx-ldb: check if channel is enabled before printing warning

If the second LVDS channel has been disabled in the DT when using dual-channel
mode we should not print a warning.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>