sched/walt: Improve the scheduler

This change is for general scheduler improvement.

Change-Id: Iffd4ae221581aaa4aeb244a0cddd40a8b6aac74d
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
[dereference23: Backport to msm-4.14]
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>