radeonsi: use optimal packet order when doing a pipeline sync

radeonsi: use optimal packet order when doing a pipeline sync

Process most new SET packets in parallel with previous draw calls, then
flush caches and wait, start the draw, and do L2 prefetches last.

This decreases the [CP busy / SPI busy] ratio (verified with GRBM perf
counters). In other words, the time window when shaders are idle (between
(the wait and the draw) is much shorter now.

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>