u_vbuf: fix vb slot assignment for translated buffers

Vertex attributes of different categories (constant/per-instance/
per-vertex) go into different buffers for translation, and this is now
properly reflected in the vertex buffers passed to the driver.

Fixes e.g. piglit's point-vertex-id divisor test.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 45ed627d89)

src/gallium/auxiliary/util/u_vbuf.c

@@ -545,6 +545,7 @@ u_vbuf_translate_find_free_vb_slots(struct u_vbuf *mgr,
 
          index = ffs(unused_vb_mask) - 1;
          fallback_vbs[type] = index;
+         unused_vb_mask &= ~(1 << index);
          /*printf("found slot=%i for type=%i\n", index, type);*/
       }
    }