drm/nouveau: drop drm_global_mutex before sleeping in submission path

If we keep hold of the mutex here, the process which currently holds the
buffer object will never be able to release it, causing a deadlock.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_gem.c

@@ -337,7 +337,9 @@ validate_init(struct nouveau_channel *chan, struct drm_file *file_priv,
 				return -EINVAL;
 			}
 
+			mutex_unlock(&drm_global_mutex);
 			ret = ttm_bo_wait_cpu(&nvbo->bo, false);
+			mutex_lock(&drm_global_mutex);
 			if (ret) {
 				NV_ERROR(dev, "fail wait_cpu\n");
 				return ret;