MIPS: tlbex: Guard tlbmiss_handler_setup_pgd

tlbmiss_handler_setup_pgd* are only referenced when
CONFIG_MIPS_PGD_C0_CONTEXT is defined.

Signed-off-by: Tony Wu <tung7970@gmail.com>
Cc: Jayachandran C. <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5601/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>