llvmpipe: don't interpolate front face or prim id

llvmpipe: don't interpolate front face or prim id

The loop was iterating over all the fs inputs and setting them
to perspective interpolation, then after the loop we were
creating extra output slots with the correct interpolation. Instead
of injecting bogus extra outputs, just set the interpolation
on front face and prim id correctly when doing the initial scan
of fs inputs.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>