sched: move definition of update_task_ravg()

set_task_cpu() will need to call update_task_ravg(). Move up
definition to make it easy.

Change-Id: I95c1c9e009bd1805f28708e8d6fd3b7b2166410e
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>