loader: Fix VK_EXT_debug_utils termination

Some of the commands aren't needed in the loader, but this would
cause command chains to crash when they hit the NULL.
Also, the ICDs expect the commands to come through vkGetInstantProcAddr
not vkGetDeviceProcAddr.  So, we need to redirect them when generating
the ICD dispatch table because the device commands in the instance
extension exist in the device dispatch table but need to call into
the ICD using the instance queries.

Change-Id: I24cd0ea4f3e5058b1137881452df75bf1ff7487a