i965: don't intepolate clip distances on pre-GEN6.

i965: don't intepolate clip distances on pre-GEN6.

On pre-GEN6 chips, the VUE slots set aside for clip distance aren't
actually used, so there is no reason for the clipper to waste time
interpolating them.

When commit 62bad54727690bff5ed42a74272e7822fd36cdb6 changed the enum
value used to represent these VUE slots, that caused the clipper to
start interpolating them as an accidental side effect.  This patch
reverts to the old clipper behavior.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>