sched/fair: reduce rounding errors in energy computations

The SG's energy is obtained by adding busy and idle contributions which
are computed by considering a proper fraction of the
SCHED_CAPACITY_SCALE defined by the SG's utilizations.

By scaling each and every contribution conputed we risk to accumulate
rounding errors which can results into a non null energy_delta also in
cases when the same total accomulated utilization is differently
distributed among different CPUs.

To reduce rouding errors, this patch accumulated non-scaled busy/idle
energy contributions for each visited SG, and scale each of them just
one time at the end.

Change-Id: Idf8367fee0ac11938c6436096f0c1b2d630210d2
Suggested-by: Joonwoo Park <joonwoop@codeaurora.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>