The iris_blorp_exec() hook needs to be executed under a single
indivisible sync region, which means that in cases where we need to
emit a PIPE_CONTROL for a buffer barrier we won't be able to track the
subsequent commands separately from the previous commands, which will
prevent us from optimizing out subsequent PIPE_CONTROLs if we
encounter the same buffers again.  In particular I've encountered this
situation in some SynMark test-cases which perform lots of BLORP
operations with the same buffer bound as both source and destination
(in order to generate mipmaps): In such a scenario if the source
requires flushing we'd also end up flushing for the destination
redundantly, even though a single PIPE_CONTROL would have been
sufficient.

This avoids a 4.5% FPS regression in SynMark OglHdrBloom and a 3.5%
FPS regression in SynMark OglMultithread.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <mesa!3875>