sched: optimize cpumask operations during task placement

Compute the CPU search mask once by taking task affinity,
cpu_active_mask and cpu_isolated_mask into account and cache
it in cpu_selection_env. This prevents doing the same cpumask
operations multiple times.

Change-Id: I78f35c59e6ee9437b3a522ac7ad856c0251f81ec
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>