drm/imx: imx-ldb: Disable both channels for split mode in enc->disable()

drm/imx: imx-ldb: Disable both channels for split mode in enc->disable()

Both of the two LVDS channels should be disabled for split mode
in the encoder's ->disable() callback, because they are enabled
in the encoder's ->enable() callback.

Fixes: 6556f7f82b9c ("drm: imx: Move imx-drm driver out of staging")
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>