powerpc/mm: Consolidate numa_enable check and min_common_depth check

If we fail to parse min_common_depth from device tree we boot with
numa disabled. Reflect the same by updating numa_enabled variable
to false. Also, switch all min_common_depth failure check to
if (!numa_enabled) check.

This helps us to avoid checking for both in different code paths.

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