imx-drm: fix parallel display regression and typo in plane format list

- Fix a regression where the parallel-display driver would not probe
  anymore if no panel is specified in the device tree, since the
  introduction of drm_of_find_panel_or_bridge.
- Fix a typo in the plane format list: replace a duplicate BGRA8888 format
  with BGRX8888, as originally intended.

* tag 'imx-drm-fixes-2017-07-18' of git://git.pengutronix.de/git/pza/linux:
  drm/imx: parallel-display: Accept drm_of_find_panel_or_bridge failure
  drm/imx: fix typo in ipu_plane_formats[]