vulkan: Stub/forwarding implementation of gpdp2 in nulldrv

Implements gpdp2 in terms of the core 1.0 functions. This is just
enough to be able to demonstrate CTS against gpdp2, and shake out
any bad loader interactions.

Where we hadn't bothered to implement the core function in nulldrv,
don't do it for gpdp2 equivalent either.

Test: dEQP-VK.api.info.get_physical_device_properties2 etc
Change-Id: I04ac05a01a67c6e03bd1a569701b046cf86877e9