Vulkan: Reduce lag on certain platforms.

After reviewing the locations where `GPU_flush()` are used it doesn't seem
to be harmfull to include these for the Vulkan backend as well. Hopefully
will save some lag that can happen when submitting one huge render graph.

Improved playback of rain_restaurant.blend where frames could be dropped
resulting into UI lag.

Pull Request: https://projects.blender.org/blender/blender/pulls/136654

source/blender/gpu/vulkan/vk_context.cc

@@ -143,7 +143,10 @@ void VKContext::end_frame()
   device.orphaned_data.destroy_discarded_resources(device);
 }
 
-void VKContext::flush() {}
+void VKContext::flush()
+{
+  flush_render_graph(RenderGraphFlushFlags::RENEW_RENDER_GRAPH);
+}
 
 TimelineValue VKContext::flush_render_graph(RenderGraphFlushFlags flags,
                                             VkPipelineStageFlags wait_dst_stage_mask,