mesa/st: fix refcnt imbalance related to st_feedback_draw_vbo()

mesa/st: fix refcnt imbalance related to st_feedback_draw_vbo()

This issue is happening on radeonsi. The reference was allocated
via _mesa_get_bufferobj_reference() with setup_arrays().
The same reference was never freed.

For instance, this issue is triggered on radeonsi with
"piglit/bin/gl-1.0-rendermode-feedback -auto -fbo"
while setting GALLIUM_REFCNT_LOG=refcnt.log.

Fixes: ff8c2a1748b9 ("mesa/bufferobj: rename bufferobj functions to be more consistent.")
Signed-off-by: Patrick Lerda <patrick9876@free.fr>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22921>