powerpc: Only use task_struct 'cpu' field on SMP

When moving to CONFIG_THREAD_INFO_IN_TASK, the thread_info 'cpu' field
gets moved into task_struct and only defined when CONFIG_SMP is set.

This patch ensures that TI_CPU is only used when CONFIG_SMP is set and
that task_struct 'cpu' field is not used directly out of SMP code.

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