loader: Fix vkGetInstanceProcAddr to handle debug_report extension

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.