powerpc/mm: Use a helper for finding pte bits mapping I/O area

Use a helper instead of open coding with constants. A later patch will
drop the WIMG bits and use PowerISA 3.0 defines.

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