Fix Win32 Unknown Device function handling

Fix Win32 Unknown Device function handling

The handling of unknown device functions queried with vkGetInstanceProcAddr
required an additional pointer dereference to get the dispatch table.

The VkDevice handle is a pointer to the `chain_device` member of
`loader_device`. This member is actually a pointer to `loader_dispatch`, or
put another way, the "start" of `loader_device`. Thus, to get access to the
dispatch table, we need to dereference the VkDevice passed into the function
then dereference the chain_device to get loader_dispatch`.