imx-drm vblank IRQ control, fence support, and of endpoint helpers

- Add and make use of drm_of_active_endpoint helpers
- Silence a noisy dev_info into a dev_dbg
- Stop touching primary fb on pageflips
- Track flip state explicitly
- Keep GEM buffer objects referenced while scanout is active
- Implement fence sync by deferring flips to a workqueue for
  dma-bufs with pending fences
- Actually disable vblank IRQs while they are not needed
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