SPU: Fix potential deadlock in event queue syscalls

[elad335]

In sys_spu_thread_receive_event/sys_event_port_send the order of mutex locking is: queue mutex then group mutex.
In sys_spu_thread_send_event the order of mutex locking is: group mutex then queue mutex.
Fix it by making sys_spu_thread_send_event not hold both mutexes at a time.