i915: Attempt to schedule buffer swap on target vertical blank when possible.

This has some advantages over the traditional way of first waiting for the
target vertical blank and then emitting the buffer swap, e.g.

* glXSwapBuffers returns immediately, only the next time the driver needs the
  hardware lock will it block until the target vertical blank. This should
  allow applications that don't intermix rendering and other processing to
  start processing for the next frame right away.
* It's less likely to produce tearing.

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