drm/imx: i.MX5 regression fix and i.MX6QP PRE/PRG stability fixes

- Disable channel burst locking on IPUv3EX (i.MX51) and IPUv3M (i.MX53).
  This fixes a regression introduced by commit 790cb4c7c954 ("drm/imx: lock
  scanout transfers for consecutive bursts").
- Give PRG a head start. Waiting for both double buffers to fill up before
  enabling the IPU improves startup reliability.
- Avoid PRE control register updates during unsafe window, workaround for
  ERR009624.
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