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.cpp b/vulkan/libvulkan/driver.cpp
index 1f1b144..17ccc72 100644
--- a/vulkan/libvulkan/driver.cpp
+++ b/vulkan/libvulkan/driver.cpp
@@ -499,7 +499,7 @@
         case ProcHook::INSTANCE:
             proc = (GetData(instance).hook_extensions[hook->extension])
                        ? hook->proc
-                       : hook->disabled_proc;
+                       : nullptr;
             break;
         case ProcHook::DEVICE:
             proc = (hook->extension == ProcHook::EXTENSION_CORE)
@@ -528,9 +528,8 @@
         return nullptr;
     }
 
-    return (GetData(device).hook_extensions[hook->extension])
-               ? hook->proc
-               : hook->disabled_proc;
+    return (GetData(device).hook_extensions[hook->extension]) ? hook->proc
+                                                              : nullptr;
 }
 
 VkResult EnumerateInstanceExtensionProperties(