MIPS: mm: Fix broken microMIPS kernel regression.

Commit f4ae17aa0f2122b52f642985b46210a1f2eceb0a [MIPS: mm: Use scratch for
PGD when !CONFIG_MIPS_PGD_C0_CONTEXT] broke microMIPS kernel builds. This
patch refactors that code similar to what was done for the 'clear_page'
and 'copy_page' functions.

Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6744/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>