i965/gen4-5: Emit MI_FLUSH as required prior to switching pipelines.

i965/gen4-5: Emit MI_FLUSH as required prior to switching pipelines.

AFAIK brw_emit_select_pipeline() is only called once during context
init on Gen4-5, at which point the pipeline is likely to be already
idle so it may just happen to work by luck regardless of the MI_FLUSH.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>