radeon: use _mesa_delete_buffer_object

This is more future-proof, plugs the memory leak of Label and properly
destroys the buffer mutex.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/dri/radeon/radeon_buffer_objects.c

@@ -71,7 +71,7 @@ radeonDeleteBufferObject(struct gl_context * ctx,
         radeon_bo_unref(radeon_obj->bo);
     }
 
-    free(radeon_obj);
+    _mesa_delete_buffer_object(ctx, obj);
 }