st/mesa: add a notify_before_flush callback param to flush

st/mesa: add a notify_before_flush callback param to flush

The new callback is called right before the flush is done to allow
users of st->flush to do some work after all the previous work has
been flushed.

This will be used by dri_flush in the next commit.

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