demos: Update cube.cpp DISPLAY_KHR code
demos: Update cube.cpp DISPLAY_KHR code

The physical display rendering code didn't compile due to drift in
`vulcan.hpp`. Update for these changes.

Tested on i.MX8M / Vivante GC7000L.

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