i965: Share the register file enum between the two backends.

i965: Share the register file enum between the two backends.

I need this so I can look at vec4 and fs registers' files from the same
.cpp file without namespaces.  As far as I can tell we never rely on the
particular numerical values of the files, though I thought it sounded like
a good idea when doing the VS (it turns out having 0 be BAD_FILE is nicer).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>