drm/bridge: fsl-ldb: Enable split mode for LVDS dual link

When LVDS dual link is used, we have to enable the LDB_CTRL_SPLIT_MODE bit.

Fixes: 463db5c2 ("drm: bridge: ldb: Implement simple Freescale i.MX8MP LDB bridge")
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Marek Vasut <marex@denx.de>
Cc: NXP Linux Team <linux-imx@nxp.com>
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220701065634.4027537-3-victor.liu@nxp.com
Link: https://patchwork.freedesktop.org/patch/msgid/20220701065634.4027537-4-victor.liu@nxp.com