powerpc: Update comments in preparation for THREAD_INFO_IN_TASK

Update a few comments that talk about current_thread_info() in
preparation for THREAD_INFO_IN_TASK.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Split out of larger patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>