[Mesa-dev] Possible Sandybridge GPU hang fixes

[Kenneth Graunke]

These patches add some missing flushing, which appears to help.  I'm
still getting GPU hangs, but they're much less frequent, and now have
an IPEHR of MI_SEMAPHORE_MBOX.  I suspect those may be due to bugs in
my performance monitoring code, rather than upstream problems.

Xinkai Chen reported that DOTA 2 used to hang every 8-10 minutes, but
after applying this patch series, it had not hung after 3 hours.

I'm not sure if these should go to stable or not.  Probably, but adding
more flushes could introduce hangs just as easily as it could fix them
(at least on Sandybridge), so I'm always nervous about that.

Patch 6 actually applies to Ivybridge, and may help with some issues
there as well.