imx-drm PRE/PRG support, deferred plane disabling, separate alpha support

- Initial support for the Prefetch Resolve Engine/Gasket on i.MX6QP,
  improving linear scanout buffer memory bandwidth utilization. This
  will in the future grow reordering support and allow direct scanout
  of Vivante tiled renderbuffers from the GPU.
- Deferred plane disabling gets rid of some busy waiting in the atomic
  plane disable and crtc disable paths that lead to wait_for_vblank
  timeouts.
- Add support for RGBA formats with a separate alpha plane, that can
  reduce memory bandwidth utilization for mostly transparent overlay
  planes by skipping color reads for completely transparent regions.
- Allow moving an active overlay plane without enforcing a modeset.
- Add 8-bit and 16-bit bayer formats to ipu_cpmem_set_image.
- Set the base address in ipu_cpmem_set_image even for invalid formats
  to increase robustness against errors.
- Use drm_plane_helper_check_state in plane atomic_check.
- Some cleanup.
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