XR_KHR_vulkan_enable2:
physicalDevice VkPhysicalDevice must match the device specified by xrGetVulkanGraphicsDevice2KHR

XR_KHR_vulkan_enable:
physicalDevice VkPhysicalDevice must match the device specified by xrGetVulkanGraphicsDeviceKHR

XR_KHR_vulkan_enable:
Add a trivial check that xrGetVulkanGraphicsDeviceKHR is called before xrCreateSession.
(our cached suggested device will be XR_NULL_HANDLE if it has not been called).
The XR_KHR_vulkan_enable2 code path already contains this check.