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
diff --git a/vulkan/libvulkan/driver_gen.h b/vulkan/libvulkan/driver_gen.h
index 1984302..ca17d57 100644
--- a/vulkan/libvulkan/driver_gen.h
+++ b/vulkan/libvulkan/driver_gen.h
@@ -49,8 +49,7 @@
     Extension extension;
 
     PFN_vkVoidFunction proc;
-    PFN_vkVoidFunction disabled_proc;  // nullptr for global hooks
-    PFN_vkVoidFunction checked_proc;   // nullptr for global/instance hooks
+    PFN_vkVoidFunction checked_proc;  // always nullptr for non-device hooks
 };
 
 struct InstanceDriverTable {