vulkan/queue: Use _mem_signal_temp instead of signal_mem_sync

The two checks should be equivalent.  This just lets us use data in
struct vk_queue_submit rather than a local boolean.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25576>

src/vulkan/runtime/vk_queue.c

@@ -976,7 +976,7 @@ vk_queue_submit(struct vk_queue *queue,
 
       vk_queue_push_submit(queue, submit);
 
-      if (signal_mem_sync) {
+      if (submit->_mem_signal_temp != NULL) {
          /* If we're signaling a memory object, we have to ensure that
           * vkQueueSubmit does not return until the kernel submission has
           * happened.  Otherwise, we may get a race between this process