i965/iris: perf-queries: don't invalidate/flush 3d pipeline

i965/iris: perf-queries: don't invalidate/flush 3d pipeline

Our current implementation of performance queries is fairly harsh
because it completely flushes and invalidates the 3d pipeline caches
at the beginning and end of each query. An argument can be made that
this is how performance should be measured but it probably doesn't
reflect what the application is actually doing and the actual cost of
draw calls.

A more appropriate approach is to just stall the pipeline at
scoreboard, so that we measure the effect of a draw call without
having the pipeline in a completely pristine state for every draw
call.

v2: Use end of pipe PIPE_CONTROL instruction for Iris (Ken)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>