i965: SF: change get_vert_attr to use the VUE map, and rename it.

i965: SF: change get_vert_attr to use the VUE map, and rename it.

The new function, called get_vert_result(), uses the VUE map to find
the register containing a given vertex attribute.  Previously, we used
the attr_to_idx[] array, which served the same purpose but didn't
account for gl_PointSize correctly.

This fixes a bug on pre-Gen6 wherein the back side of a triangle would
be rendered incorrectyl if the vertex shader wrote to gl_PointSize.

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