st/mesa: remove redundant flushes from st_flush

st/mesa: remove redundant flushes from st_flush

st_flush should flush state tracker-internal state and the pipe, but
not mesa/main state. Of the four callers:

- glFlush/glFinish already call FLUSH_{VERTICES,STATE}.
- st_vdpau doesn't need to call them.
- st_manager will now call them explicitly.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>