i965/vs: Convert gen6 userclip handling to new generators.
i965/vs: Convert gen6 userclip handling to new generators.

This DP4 had one of its operands missing, so we were generating
garbage clip distances.  Using the per-opcode instruction generators
made it obvious.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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