r600g: atomize streamout enabling

r600g: atomize streamout enabling

This doesn't fix any issue we know of, but there indeed is a week spot
in draw_vbo where streamout can fail. After streamout is enabled,
the need_cs_space call can flush the context, which causes the streamout
to be disabled right after it was enabled and bad things happen.

One way to fix it is to atomize the beginning part, so that no context flush
can happen between streamout enabling and the first drawing.

Tested-by: Andreas Boll <andreas.boll.dev@gmail.com>