gallium/u_threaded: set has_user_indices = false for merged draws

Fixes an assert when running this piglit test:
   arb_shader_draw_parameters-drawid-single-draw

Fixes: 351ba767af ("gallium/u_threaded: set has_user_indices = false in the driver thread")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8004>

src/gallium/auxiliary/util/u_threaded_context.c

@@ -98,6 +98,7 @@ simplify_draw_info(struct pipe_draw_info *info)
    /* Clear these fields to facilitate draw merging.
     * Drivers shouldn't use them.
     */
+   info->has_user_indices = false;
    info->index_bounds_valid = false;
    info->_pad = 0;