[Intel-gfx] Improvements and fixes for Intel ddx swap scheduling/timestamping.

[Mario Kleiner]

Hi,

a series of three patches to improve the dri2 swap scheduling
and timestamping for the current intel ddx.

The first one enables proper OML_sync_control timestamping
while triple-buffering is enabled and XOrg 1.12+ with DRI2SwapLimit
support is in use. So far, timestamping was unuseable with
triple-buffering, only worked with double-buffering.

The second one repairs the broken pageflip swap scheduling, which
is apparently in a frightening state for timing sensitive apps
since a year, due to a tiny but really ugly bug. In a perfect
implementation of Murphy's law, the same commit that broke the
scheduling also disabled the builtin correctness checks that
were supposed to catch such bugs.

The third one proposes to revert 'SwapBuffersWait' to its old
behaviour where it didn't affect pageflipping. I just can't
think of a case where the current behaviour makes any sense, not
even for benchmarking? But maybe i'm overlooking something.

All patches were tested against an Intel 945-GME gpu.

I don't really care about the 'SwapBuffersWait' patch one way
or the other, but the first two are crucial to make the intel
ddx useable in a painless and safe way again for users of
timing sensitive applications.

Thanks,
-mario