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 <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp

@@ -1858,9 +1858,9 @@ vec4_visitor::emit_vue_header_gen6(int header_mrf)
 	 else
 	    m = brw_message_reg(header_mrf + 1);
 
-	 emit(BRW_OPCODE_DP4,
-	      dst_reg(brw_writemask(m, 1 << (i & 3))),
-	      src_reg(c->userplane[i]));
+	 emit(DP4(dst_reg(brw_writemask(m, 1 << (i & 3))),
+		  src_reg(output_reg[VERT_RESULT_HPOS]),
+		  src_reg(c->userplane[i])));
       }
       header_mrf += 2;
    }