mmu-hash*: Cleanup segment-level NX check

On the ppc hash mmus, no-execute can be set at the segment level (on more
recent 64-bit hash mmus it can also be set at the page level).  This patch
separates out this check to make it clearer what is going on, and avoiding
excessive indentation of the remaining translation code.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
