scripts: Re-added AllPlatforms wsi option
scripts: Re-added AllPlatforms wsi option

A single cross-platform dispatch table helper file is preferable.
Added the AllPlatforms parameter back into vk-generate.py

Change-Id: Icc00c04504d58007270eb55fc269651c215aa9b4

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