drm/imx: imx-drm ldb and ipu-v3 csi fixes

- Disable the LVDS Display Bridge (LDB) on driver bind. This is
  necessary to guarantee correct LVDS signals in case the bootloader
  left the LVDS output active.
- Remove false positive warning about disabled second LVDS channel in
  dual-channel mode. In this mode, the second LVDS channel can not be
  used separately. If the second channel is correctly described as
  disabled in the device tree, the driver warned about this anyway.
- Fix the CSI confiuration to not only enable interlaced capture mode
  for V4L2_FIELD_SEQ_BT and V4L2_FIELD_SEQ_TB, but also for the
  V4L2_FIELD_ALTERNATE interlacing mode. Before, it incorrectly tried
  to capture progressive frames in that case.
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