r600g: align flushing of cb/db with DDX/r600c.

r600g: align flushing of cb/db with DDX/r600c.

the DDX and r600c both flush cb/db after the draw is emitted,
as long as they do that, r600g can't be different, as it races.

We end up with r600g flush, set CB, DDX set CB, flush. This
was causing misrendering on my evergreen, where sometimes the drawing
would go to an old CB.