nvk: Do a second submit to check for errors in the sync case

With NVK_DEBUG=push_sync, we signal and immediately wait on a syncobj
to better detect whitch push failed and then print it.  However, the
syncobj wait itself isn't enough to check for errors.  We have to try
pushing again.  Adding an empty push lets us get errors immediately and
print the actually failing pushbuf to stderr.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27320>

src/nouveau/vulkan/nvk_queue_drm_nouveau.c

@@ -269,6 +269,18 @@ push_submit(struct push_builder *pb, struct nvk_queue *queue, bool sync)
          return vk_errorf(queue, VK_ERROR_UNKNOWN,
                           "DRM_SYNCOBJ_WAIT failed: %m");
       }
+
+      /* Push an empty again, just to check for errors */
+      struct drm_nouveau_exec empty = {
+         .channel = pb->req.channel,
+      };
+      err = drmCommandWriteRead(pb->dev->ws_dev->fd,
+                                DRM_NOUVEAU_EXEC,
+                                &empty, sizeof(empty));
+      if (err) {
+         return vk_errorf(queue, VK_ERROR_DEVICE_LOST,
+                          "DRM_NOUVEAU_EXEC failed: %m");
+      }
    }
    return VK_SUCCESS;
 }