sched: kill unnecessary divisions on fast path

The max_possible_efficiency and CPU's efficiency are fixed values which
are determined at cluster allocation time.  Avoid division on the fast
by using precomputed scale factor.

Also update_cpu_busy_time() doesn't need to know how many full windows
have elapsed.  Thus replace unneeded division with simple comparison.

Change-Id: I2be1aad3fb9b895e4f0917d05bd8eade985bbccf
Suggested-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>