[Intel-gfx] [RFC] vblank worker for asynchronous unpinning and other tasks

[Chris Wilson]

Just a small series of patches that I like to get some feedback on.

The central goal of the series is to enable asynchronous sprite updates.
However, being able to asynchronously queue work to take place after the
next vblank is useful elsewhere so I have tried to generalise and find
some other users.

Please review,
-Chris