layers: Refactor CmdBindDescSets for pre/post
layers: Refactor CmdBindDescSets for pre/post

Validation and state updates are now somewhat less tangled.

Change-Id: Ia666215a73ce02d2fbc326fb5b37b3361d83bbe3

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