sched/fair: Remove task_util from effective utilization in feec()

The energy estimation in find_energy_efficient_cpu() (feec()) relies on
the computation of the effective utilization for each CPU of a perf domain
(PD). The function effective_cpu_util() which gives this value, scales the
utilization relative to IRQ pressure on the CPU to take into account that
the IRQ time is hidden from the task clock. The IRQ scaling is as follow:

   effective_cpu_util = irq + (cpu_cap - irq)/cpu_cap * util

Where util is the sum of CFS/RT/DL utilization, cpu_cap the capacity of
the CPU and irq the IRQ avg time.

If now we take as an example a task placement which doesn't raise the OPP
on the candidate CPU, we can write the energy delta as:

  delta = OPPcost/cpu_cap * (effective_cpu_util(cpu_util + task_util) -
                             effective_cpu_util(cpu_util))
        = OPPcost/cpu_cap * (cpu_cap - irq)/cpu_cap * task_util

We end-up with an energy delta depending on the IRQ avg time, which is a
problem: first the time spent on IRQs by a CPU has no effect on the
additional energy that would be consumed by a task. Second, we don't want
to favour a CPU with a higher IRQ avg time value.

Nonetheless, we need to take the IRQ avg time into account. If a task
placement raises the PD's frequency, it will increase the energy cost for
the entire time where the CPU is busy. A solution is to only use
effective_cpu_util() with the CPU contribution part. The task contribution
is added separately and scaled according to prev_cpu's IRQ time.

No change for the FREQUENCY_UTIL component of the energy estimation. We
still want to get the actual frequency that would be selected after the
task placement.

Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>