imx-drm mode fixup support, imx-hdmi bridge conversion and imx-drm cleanup

- Implement mode_fixup for a DI vertical timing limitation
- Use generic DRM OF helpers in DRM core
- Convert imx-hdmi to dw_hdmi drm_bridge and add rockchip
  driver
- Add DC use counter to fix multi-display support
- Simplify handling of DI clock flags
- A few small fixes and cleanup
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