powerpc/mm: Don't use pmd_val, pud_val and pgd_val as lvalue

We convert them static inline function here as we did with pte_val in
the previous patch

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