i965: Prepare for using the ATTR register file in the fs backend

i965: Prepare for using the ATTR register file in the fs backend

The scalar vertex shader will use the ATTR register file for vertex
attributes.  This patch adds support for the ATTR file to fs_visitor.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>