imx-drm updates and encoder atomic_mode_set helper callback

- add pixel clock and DE polarity configuration from device tree
  using display timing bindings for parallel and LVDS output
- cleanup/remove trivial functions
- cleanup and fixes in preparation for capture support
- add atomic_mode_set helper and use it in imx-ldb - this is an
  alternative to the encoder mode_set callback that passes the
  crtc and connector state instead of just the mode. It allows
  drivers to get information from the attached connector without
  having to iterate over all connectors
- add drm_bridge support to imx-ldb, for bridges attached via LVDS
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