crimson/osd/scheduler: add immediate class check in scheduler wrapper

ClassedOpQueueScheduler (WPQ) does not handle SchedulerClass::immediate
in get_priority() -- it asserts klass < immediate.

Solution: Fix by checking for immediate class before calling
get_priority() in both enqueue() and enqueue_front().

Signed-off-by: Mohit Agrawal <mohit84.agrawal@gmail.com>