sched/boost: provide helper api for boost on big

Currently the two callsites check if a task has sched boost enabled
and if the boost policy is "boost on big". Provide a helper api
to do the needful checks. This will further help with improving
colocation boost in the upcoming patch.

Change-Id: Ia55e4fe6cae66d7c7883f8a3c0a5d851a7917fc8
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>