r300: fix vertex_buffer related refcnt imbalance

Indeed, vertex_buffer was not properly freed.

For instance, this issue is triggered with:
"piglit/bin/fcc-read-after-clear blit rb -auto -fbo"
while setting GALLIUM_REFCNT_LOG=refcnt.log.

Fixes: 8a963d122d ("r300g/swtcl: don't do stuff which is only for HWTCL")
Signed-off-by: Patrick Lerda <patrick9876@free.fr>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27678>
(cherry picked from commit 3b90c46bdf)

.pick_status.json

@@ -2654,7 +2654,7 @@
         "description": "r300: fix vertex_buffer related refcnt imbalance",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "8a963d122d4b67209214b0483084ba950814f42b",
         "notes": null

src/gallium/drivers/r300/r300_context.c

@@ -86,6 +86,9 @@ static void r300_destroy_context(struct pipe_context* context)
     if (r300->draw)
         draw_destroy(r300->draw);
 
+    for (unsigned i = 0; i < r300->nr_vertex_buffers; i++)
+       pipe_vertex_buffer_unreference(&r300->vertex_buffer[i]);
+
     if (r300->uploader)
         u_upload_destroy(r300->uploader);
     if (r300->context.stream_uploader)