ANDROID: sched/fair: fix place_entity() vendor hook

place_entity() vendor hook is meant to tweak vruntime by vendor
modules as needed, but with current form of the hook that is not
possible as vruntime is passed by it's value. Fix it by switching
to pass by reference.

Bug: 175448877
Change-Id: Ibb51592f94da31019fa98a6767d080ec61daafe6
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>