zink: always force flushes when originating from api frontend

flags=0 is used for e.g., glFenceSync, which apps use to insert sync points
to determine when all prior work has completed. eliding these flushes into no-ops
is fine for all scenarios except when the last op was a present, in which
case the no-op (previous) fence will not sync as expected for the present and
graphical artifacts will result

in the future, this may be changed back to the previous behavior if/when presentation
gains timeline semaphore capabilities by providing the last timeline id
as a fence instead of the last batch

fixes #10386

cc: mesa-stable

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26935>

src/gallium/drivers/zink/zink_context.c

@@ -3863,7 +3863,8 @@ zink_flush(struct pipe_context *pctx,
       }
    }
 
-   if (!batch->has_work) {
+   /* TODO: if swapchains gain timeline semaphore semantics, `flags` can be eliminated and no-op fence can return timeline id */
+   if (!batch->has_work && flags) {
        if (pfence) {
           /* reuse last fence */
           fence = ctx->last_fence;