r600g: Flush upload buffers before draws instead of before flushes.

r600g: Flush upload buffers before draws instead of before flushes.

If a upload buffer is used by a previous draw that's still in the CS,
accessing it would need a context flush. However, doing a context flush when
mapping the upload buffer would then flush/destroy the same buffer we're trying
to map there. Flushing the upload buffers before a draw avoids both the CS
flush and the upload buffer going away while it's being used. Note that
u_upload_data() could e.g. use a pool of buffers instead of allocating new
ones all the time if that turns out to be a significant issue.