draw: fix clipping with linear interpolated values and gl_ClipVertex

draw: fix clipping with linear interpolated values and gl_ClipVertex

Discovered this when working on other clip code, apparently didn't work
correctly - the combination of linear interpolated values and using
gl_ClipVertex produced wrong values (failing all such combinations
in piglits glsl-1.30 interpolation tests, named
interpolation-noperspective-XXX-vertex).
Use the pre-clip-pos values when determining the interpolation factor to
fix this.
Noone really understands this code well, but everybody agrees this looks
sane... This fixes all those failing tests (10 in total) both with
the llvm and non-llvm draw paths, with no piglit regressions.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>