i965: Throttle rendering to an fbo

When rendering to an fbo, even though it may be acting as a winsys
frontbuffer or just generally, we never throttle. However, when rendering
to an fbo, there is no natural frame boundary. Conventionally we use
SwapBuffers and glFinish, but potential callers avoid often glFinish for
being too heavy handed (waiting on all outstanding rendering to complete).
The kernel provides a soft-throttling option for this case that waits for
rendering older than 20ms to be complete (that's a little too lax to be
used for swapbuffers, but is here a useful safety net). The remaining
choice is then either never to throttle, throttle after every draw call,
or at after intermediate user defined point such as glFlush and thus all the
implied flushes. This patch opts for the latter as that is the current
method used for flushing to front buffers.

v2: Defer the throttling from inside the flush to the next
intel_prepare_render() and switch non-fbo frontbuffer throttling over to
use the same lax method. The issuing being that
glFlush()/intel_prepare_read() is just as likely to be called inside a
tight loop and not at "frame" boundaries.

v3: Rename from need_front_throttle to need_flush_throttle to avoid any
ambiguity between front buffer rendering and fbo rendering. (Chad)

v4: Whitespace

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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