i965: Move BAD_FILE from the beginning of enum register_file.

i965: Move BAD_FILE from the beginning of enum register_file.

I'm going to begin using brw_reg's file field in backend_reg and its
derivatives, and in order to keep the hardware value for ARF as 0, we
have to do something different.

Reviewed-by: Emil Velikov <emil.velikov@collabora.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>