target-ppc: Rework get_physical_address()

Currently get_physical_address() first checks to see if translation is
enabled in the MSR, then in the translation on case switches on the mmu
type.  Except that for BookE MMUs, translation is always on, and so it
has to switch in the "translation off" case as well and do the same thing
as the translation on path for those MMUs.  Plus, even translation off
doesn't behave exactly the same on the various MMU types so there are
further mmu type checks in the "translation off" path.

As a first step to cleaning this up, this patch moves the switch on mmu
type to the top level, then makes the translation on/off check just for
those mmu types where it is meaningful.

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