layers: Fix object tracker memory leak
layers: Fix object tracker memory leak

Upon repeated calls, GetDisplayPlaneSupportedDisplaysKHR will call
CreateObject multiple times with the same VkDisplayKHR handle.  Add a
check in CreateObject for an existing tracking object before allocating
a new one.

Change-Id: I50f634fbe8b7e69edbfeb4e6f0c050b7fccaa6e4

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