sched: fix incorrect prev_runnable_sum accounting with long ISR run

At present, when IRQ handler spans multiple scheduler windows, HMP
scheduler resets the IRQ CPU's prev_runnable_sum with its current max
capacity under the assumption that there is no other possible contribution
to the CPU's prev_runnable_sum.  This isn't correct as another CPU can
migrate tasks to the IRQ CPU.

Furthermore such incorrectness can trigger BUG_ON() if the migrated task's
prev_window is larger than migrating CPU's current capacity in following
scenario.

1. ISR on the power efficient CPU has been running for multiple windows.
2. A task which has prev_window higher than IRQ CPU's current capacity
   migrated to the IRQ CPU.
3. Servicing IRQ is done and the IRQ CPU resets its prev_runnable_rum =
   CPU's current capacity.
4. Before window rollover, the task on the IRQ CPU migrates to other CPU
   and fixes up source and destnation CPUs' busy time.
5. BUG_ON(src_rq->prev_runnable_sum < 0) triggers as p->ravg.prev_window
   is larger than src_rq->prev_runnable_sum.

Fix such incorrectness by preserving prev_runnable_sum when ISR spans
multiple scheduler windows.  There is no need to reset it.

CRs-fixed: 828055
Change-Id: I1f95ece026493e49d3810f9c940ec5f698cc0b81
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>