i965: Fix an inconsistency inb the VUE map with gl_ClipVertex on gen4/5.

i965: Fix an inconsistency inb the VUE map with gl_ClipVertex on gen4/5.

We are intentionally not allocating a slot for gl_ClipVertex.  But by
leaving the bit set in the slots_valid, the fragment shader's computation
of where varyings are in urb entry coming out of the SF would be off by
one.  Fixes rendering in Freespace 2 SCP, and improves rendering in TF2.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=62830
Tested-by: Joaquín Ignacio Aramendía <samsagax@gmail.com>
NOTE: This is a candidate for the 9.1 branch.
Reviewed-and-tested-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>