mm/MIPS: use free_highmem_page() to free highmem pages into buddy system

Use helper function free_highmem_page() to free highmem pages into
the buddy system.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: David Daney <david.daney@cavium.com>
Cc: Cong Wang <amwang@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>