ppc: do not look at the MMU index to detect PR/HV mode

The MMU index is an internal detail that should not be needed by the
translator (except to generate loads and stores).  Look at the MSR
directly.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>