i965/gen6-7: Implement stall and flushes required prior to switching pipelines.

i965/gen6-7: Implement stall and flushes required prior to switching pipelines.

Switching the current pipeline while it's not completely idle or the
read and write caches aren't flushed can lead to corruption.  Fixes
misrendering of at least the following Khronos CTS test:

 ES31-CTS.shader_image_load_store.basic-allTargets-store-fs

The stall and flushes are no longer required on Gen8+.

v2: Emit PIPE_CONTROL with non-zero post-sync op before the write
    cache flush on SNB due to hardware bug. (Ken)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93323
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>