i965/fs: Make register file enum 0 be the undefined register file.

i965/fs: Make register file enum 0 be the undefined register file.

In 6d874d0ee18b3694c49e0206fa519bd8b746ec24, I checked whether a
register that had been stored was BAD_FILE (as opposed to a legitimate
GRF), but actually the unset register was ARF NULL because it had been
memset to 0.  Finding BAD_FILE for unset values in debugging was my
intention with that file, so make it the case more often by
rearranging the enum.  There was only one place we relied on the magic
enum register_file to hardware register file correspondance anyway.