drm/imx: cleanup and csi improvements

- Remove the unused struct imx_drm_crtc and the unused pipes field
  from imx_drm_device and replace drm_dev_unref with drm_dev_put.
- Extend CSI configuration to support RGB888 and BGR888 capture,
  as well as 16-bit RGB565 capture via a parallel bus.
- Add CPMEM support for negative interlace offsets, which is
  necessary to support writing captured bottom-top interlaced
  fields to memory with interleaved lines.
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