The commit a923660d786a53e78834b19062f7af2535f7f8ad accidently
prevents TX49 from using CDEX.  Use build_dst_pref() only if prefetch
for store was really available.
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>