[MIPS] Fix warning by moving do_default_vi into CONFIG_CPU_MIPSR2_SRS

This fixes the warning:

arch/mips/kernel/traps.c:931: warning: 'do_default_vi' defined but not used

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>