r600: fix refcnt imbalance related to r600_set_vertex_buffers()
For instance, this issue is triggered with: "piglit/bin/useprogram-flushverts-2 -auto -fbo" or "piglit/bin/primitive-restart-draw-mode line_loop -auto" while setting GALLIUM_REFCNT_LOG=refcnt.log.
cc: mesa-stable
Signed-off-by: Patrick Lerda patrick9876@free.fr