frontends/va: Fix SyncSurface when used to sync coded buffer

This would skip the coded buffer fence wait if the surface fence is NULL.

Fixes: 0f20a3a4f1 ("frontends/va: Add surface pipe_fence for vl_compositor rendering")
Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35782>

src/gallium/frontends/va/surface.c

@@ -188,7 +188,7 @@ _vlVaSyncSurface(VADriverContextP ctx, VASurfaceID render_target, uint64_t timeo
    }
 
    /* No outstanding operation: nothing to do. */
-   if (!surf->fence) {
+   if (!fence) {
       mtx_unlock(&drv->mutex);
       return VA_STATUS_SUCCESS;
    }