i965/fs: Remove one more fixed brw_null_reg() from the visitor.

Instead use fs_builder::null_reg_f() which has the correct register
width.  Avoids the assertion failure in fs_builder::emit() hit by the
"ES3-CTS.shaders.loops.for_dynamic_iterations.unconditional_break_fragment"
GLES3 conformance test introduced by 4af4cfba9e.

Reported-and-reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

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

@@ -3234,7 +3234,7 @@ fs_visitor::lower_integer_multiplication()
          ibld.ADD(dst, low, high);
 
          if (inst->conditional_mod) {
-            fs_reg null(retype(brw_null_reg(), inst->dst.type));
+            fs_reg null(retype(ibld.null_reg_f(), inst->dst.type));
             set_condmod(inst->conditional_mod,
                         ibld.MOV(null, inst->dst));
          }