sched/fair: Rework feec() to use cost instead of spare capacity

feec() looks for the CPU with highest spare capacity in a PD assuming that
it will be the best CPU from a energy efficiency PoV because it will
require the smallest increase of OPP. Although this is true generally
speaking, this policy also filters some others CPUs which will be as
efficients because of using the same OPP.
In fact, we really care about the cost of the new OPP that will be
selected to handle the waking task. In many cases, several CPUs will end
up selecting the same OPP and as a result using the same energy cost. In
these cases, we can use other metrics to select the best CPU for the same
energy cost.

Rework feec() to look 1st for the lowest cost in a PD and then the most
performant CPU between CPUs.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>