loader: Use ICD's GetDeviceProcAddr as end of layer chain rather than loader's
loader: Use ICD's GetDeviceProcAddr as end of layer chain rather than loader's

If function name is unknown to loader and layers the old gpa_internal would
recurse without terminator. Use Driver's GPA as terminator.
Also simplifies code.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools