powerpc/64s: Idle do not hold reservation longer than required

When taking the core idle state lock, grab it immediately like a regular
lock, rather than adding more tests in there. Holding the lock keeps it
stable, so there is no need to do it whole holding the reservation.

Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>