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

Change-Id: I78f9c5ecb36edbd2515f7ef2a889519e45707ea8

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools