powerpc/mm: Use pmd_lockptr instead of opencoding it

In later patch we switch pmd_lock from mm->page_table_lock to split pmd ptlock.
It avoid compilations issues, use pmd_lockptr helper.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>