vulkan: fix vkGet*ProcAddr for un-enabled extensions

vulkan::api::InitDispatchTable no longer queries for non-enabled WSI
functions.  We could now return NULL instead of ProcHook::disabled_proc.
This also matches what the spec says.

Bug: 28173232
Change-Id: I05c45303025d25e49f75c18a912fc4cc2b13979f