Reduce time lock in psci_do_cpu_off

psci_set_power_off_state only initializes a local variable, so there
isn't any reason why it should be done while the lock is held.

Change-Id: I1c62f4cd5d860d102532e5a5350152180d41d127
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>