turnip: lower device index to zero

Another simple issue I spotted while looking through CTS failures.

Vulkan 1.1 has VK_KHR_device_group and VK_KHR_device_group_creation promoted to core, thus we should handle DeviceIndex built-in.

While we are here, also add these extensions to the extensions list, even though they are not doing anything useful.

Fixes test:

 dEQP-VK.compute.device_group.device_index