sched/fair: call OPP update when going idle after migration

When a task leaves a rq because it is migrated away it carries its
utilization with him. In this case and OPP update on the src rq might be
needed. The corresponding update at dst rq will happen at enqueue time.

Change-Id: I22754a43760fc8d22a488fe15044af93787ea7a8

sched/fair: Fix uninitialised variable in idle_balance

compiler warned, looks legit.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>