sched: walt: Fix irq-work load reporting logic

Previously, the irq-work was not being scheduled because
of an uninit'ed atomic and the fact that windows may roll
over outside of the scheduler_tick() function.

Therefore, move the scheduling of the irq-work into the
update_task_ravg() function, and init the global atomic
the first time that the atomic compare-exchange results
in a zero.

Change-Id: If1e4d7d48143bc8dd70425be97c629df588e5625
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>