Cycles: Make sure GPU transfer is finished prior display update

Noticed while looking into flickering issues in viewport.

Doesn't seem to solve the flicker issue for me, but is something
what is supposed to be happening anyway.

Differential Revision: https://developer.blender.org/D12673

intern/cycles/integrator/path_trace_work_gpu.cpp

@@ -738,7 +738,8 @@ void PathTraceWorkGPU::copy_to_gpu_display_naive(GPUDisplay *gpu_display,
 
   get_render_tile_film_pixels(destination, pass_mode, num_samples);
 
-  gpu_display_rgba_half_.copy_from_device();
+  queue_->copy_from_device(gpu_display_rgba_half_);
+  queue_->synchronize();
 
   gpu_display->copy_pixels_to_texture(
       gpu_display_rgba_half_.data(), texture_x, texture_y, width, height);