anv/radv: Resolving 'GetInstanceProcAddr' should not require a valid instance

anv/radv: Resolving 'GetInstanceProcAddr' should not require a valid instance

Since vk_icdGetInstanceProcAddr() is wired through
vkGetInstanceProcAddr() in both drivers, we lost the ability for
'GetInstanceProcAddr' to resolve itself prior to having a valid
instance.

An upcoming spec change will fix that and allow
vkGetInstanceProcAddr() to resolve itself passing NULL as
instance. See https://gitlab.khronos.org/vulkan/vulkan/issues/2057
for details.

This patch implements the change in both radv and anvil.

CTS changes have already landed:
https://gitlab.khronos.org/Tracker/vk-gl-cts/issues/2278

vulkan-loader changes have also landed:
https://gitlab.khronos.org/Tracker/vk-gl-cts/issues/2278

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4273>