drm/imx: imx-ldb: Register LDB channel1 when it is the only channel to be used

LDB channel1 should be registered if it is the only channel to be used.
Without this patch, imx_ldb_bind() would skip registering LDB channel1
if LDB channel0 is not used, no matter LDB channel1 needs to be used or
not.

Fixes: 8767f471 (drm/imx: imx-ldb: move initialization into probe)
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>