drm/imx: fixes for early vblank event issue, array underflow error

- fix an array underflow error by reordering the range check before the array
  subscript in ipu-prg.
- make some local functions static in ipuv3-plane.
- add a missng header for ipu_planes_assign_pre in ipuv3-plane.
- move arming of the vblank event from atomic_begin to atomic_flush, to avoid
  signalling atomic commit completion to userspace before plane atomic_update
  has finished, due to a race condition that is likely to be hit on i.MX6QP on
  PRE enabled channels.
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