llvmpipe/query: add support for partial query waits.

Without overlap, the fence should always be signalled so this
won't be hit, but once overlap is enabled, then cases will start
to fail due to missing this.

Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14923>

src/gallium/drivers/llvmpipe/lp_query.c

@@ -238,6 +238,10 @@ llvmpipe_get_query_result_resource(struct pipe_context *pipe,
    else {
       unsigned i;
 
+      /* don't write a value if fence hasn't signalled,
+         and partial isn't set . */
+      if (unsignalled && !(flags & PIPE_QUERY_PARTIAL))
+         return;
       switch (pq->type) {
       case PIPE_QUERY_OCCLUSION_COUNTER:
          for (i = 0; i < num_threads; i++) {