i965: Throttle to the previous frame

In order to facilitate the concurrency offered by triple buffering and to
offset the latency induced by swapping via an external process, which
may incur extra rendering itself, only throttle to the previous frame
and not the last. The second issue that mostly affects swap benchmarks,
but also can incur jitter in the throttling, is that the throttle bo is
closer to the next SwapBuffers rather than immediately after the previous
SwapBuffers. Throttling to the previous frame doubles the maximum possible
latency at the benefit of improving throughput and reducing jitter.

v2: Rename "first_post_swapbuffer" batches array to a plain
throttle_batch[] as the pluralisation was contorting the name and not
making it clear as to whether it was the first batch or first_post_swap
batch. Not least of which was that not all throttle points are SwapBuffers.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Kenneth Graunke <[email protected]>
Cc: Ben Widawsky <[email protected]>
Cc: Kristian Høgsberg <[email protected]>
Cc: Chad Versace <[email protected]>
Cc: Ian Romanick <[email protected]>
Reviewed-by: Chad Versace <[email protected]>

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