drm/imx: handle pending updates better, add plane zpos property support

- Add a mechanism to only send commit done events once all pending
  updates have been applied. This closes a small race window where
  already armed events could fire even though the double buffered
  hardware update just missed the update window.
- Add plane zpos property support to allow placing the overlay plane
  behind the primary plane.
- Allow building imx-drm on all platforms under COMPILE_TEST.
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