loader: Fix vkGetInstanceProcAddr to handle debug_report extension

Need loader entrypoints for debug_report extension including the utility
functions. Don't call down the instance chain GPA for this extension.

Remove instance extensions decoding when GPA instance == NULL as don't want
to return extension entrypoints unless they are enabled.
This meant the WSI swapchain instance GPA was no longer used so remove it.
diff --git a/loader/debug_report.h b/loader/debug_report.h
index a04386a..7b008d3 100644
--- a/loader/debug_report.h
+++ b/loader/debug_report.h
@@ -98,9 +98,10 @@
         struct loader_instance *ptr_instance,
         const VkInstanceCreateInfo *pCreateInfo);
 
-void *debug_report_instance_gpa(
+bool debug_report_instance_gpa(
         struct loader_instance *ptr_instance,
-        const char* name);
+        const char* name,
+        void **addr);
 
 VkResult VKAPI loader_DbgCreateMsgCallback(
     VkInstance                          instance,