sched: walt: use_cycle_counter check before rq lock held

use_cycle_counter is not necessary need held
rq lock, so check use_cycle_counter before rq
lock in sched_account_irqstart to eliminate
unnecessary rq lock and unlock.

Change-Id: Ie0f3d87b746dffbb418dd5d8b2e1892adac2ba16
Signed-off-by: Maria Yu <aiquny@codeaurora.org>