KVM: PPC: Book3S: PR: Enable interrupts earlier

Now that the svcpu sync is interrupt aware we can enable interrupts
earlier in the exit code path again, moving 32bit and 64bit closer
together.

While at it, document the fact that we're always executing the exit
path with interrupts enabled so that the next person doesn't trap
over this.

Signed-off-by: Alexander Graf <agraf@suse.de>