sched: fix compilation errors with !SCHED_HMP

Accessing p->grp in __migrate_task() and detach_task()
breaks compilation when SCHED_HMP is not defined. Add a
wrapper to check if a task belongs to related thread
group or not.

Change-Id: I558f9a6c7b5f9bd4a21b5021b8eb80bace69bc32
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>