i965: SF: Change the flags that refer to "attr" to be based on gl_vert_result.

i965: SF: Change the flags that refer to "attr" to be based on gl_vert_result.

Previously, some of the code in SF erroneously used bitfields based on
the gl_frag_attrib enum when actually referring to vertex results.
This worked, because coincidentally the particular enum values being
used happened to match between gl_frag_attrib and gl_vert_result.  But
it was fragile, because a future change to either gl_vert_result or
gl_frag_attrib would have made the enum values stop matching up.  This
patch switches the SF code to use the correct enum.

Reviewed-by: Eric Anholt <eric@anholt.net>