i915: Fix use of uninitialized variable in OPCODE_NOISE stub.

We don't actually care which register is used since we're just
swizzling (0,0,0,0), but it should be a valid variable number.
Detected by clang.

src/mesa/drivers/dri/i915/i915_fragprog.c

@@ -663,7 +663,7 @@ upload_program(struct i915_fragment_program *p)
 			 A0_MOV,
 			 get_result_vector(p, inst),
 			 get_result_flags(inst), 0,
-			 swizzle(src0, ZERO, ZERO, ZERO, ZERO), 0, 0);
+			 swizzle(tmp, ZERO, ZERO, ZERO, ZERO), 0, 0);
 
       case OPCODE_POW:
          src0 = src_vector(p, &inst->SrcReg[0], program);