sched: support legacy mode better

It should be possible to bypass all HMP scheduler changes at runtime
by setting sysctl_sched_enable_hmp_task_placement and
sysctl_sched_enable_power_aware to 0.  Fix various code paths to honor
this requirement.

Change-Id: I74254e68582b3f9f1b84661baf7dae14f981c025
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>