demos: Fix access masks when cube uses staging buffer
demos: Fix access masks when cube uses staging buffer

Change-Id: I89655f8224c3543d9c57f2cf7bd55b2c2a21c09f

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers