i965: Refactor Gen6+ SF attribute override code.

The next patch will benefit from easy access to the source attribute
number and whether or not we're swizzling.  It doesn't want the final
attr_override DWord form, however.

Reviewed-by: Paul Berry <stereotype441@gmail.com>
Tested-by: Martin Steigerwald <martin@lichtvoll.de>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit b3efc5bea85a84493a4b81eea0d3c3acfe42813f)
1 file changed