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

* tag 'imx-drm-next-20160301' of git://git.pengutronix.de/git/pza/linux:
  drm/imx: only enable vblank IRQs when needed
  drm/imx: implement fence sync
  drm/imx: keep GEM object referenced as long as scanout is active
  drm/imx: track flip state explicitly
  drm/imx: don't touch primary fb on pageflip
  drm/imx: ipuv3 plane: Replace dev_info with dev_dbg if a plane's CRTC changes
  gpu: ipu-v3: ipu-dc: Simplify display controller microcode setup
  drm/rockchip: remove rockchip_drm_encoder_get_mux_id
  drm/imx: remove imx_drm_encoder_get_mux_id
  drm: add drm_of_encoder_active_endpoint helpers