draw: flush when setting stream-out targets
We need to re-prepare the middle-end state to pick up changes to this state to react correctly to pausing/resuming stream-out. So let's add a flush here.
Signed-off-by: Erik Faye-Lund erik.faye-lund@collabora.com Fixes: ec8cbd79 "draw/softpipe: EXT_transform_feedback support (v2)"