sched: EAS: Fix the condition to distinguish energy before/after

Before commit 5f8b3a757d65 ("sched/fair: consider task utilization in
group_norm_util()"), eenv->util_delta is used to distinguish energy
before and energy after in sched_group_energy(). After that commit,
eenv->util_delta can not do that any more.

In this commit, use trg_cpu to distinguish energy before/after in
sched_group_energy().

Before apply this commit, cap_before/cap_delta is not correct:
<idle>-0 [001] 147504.608920: sched_energy_diff: pid=7 comm=rcu_preempt
src_cpu=1 dst_cpu=3 usage_delta=7 nrg_before=250 nrg_after=250 nrg_diff=0
cap_before=0 cap_after=528 cap_delta=1056 nrg_delta=0 nrg_payoff=0

After apply this commit, cap_before/cap_delta retrun to normal:
<idle>-0 [001] 220.494011: sched_energy_diff:    pid=7 comm=rcu_preempt
src_cpu=1 dst_cpu=2 usage_delta=3 nrg_before=248 nrg_after=248 nrg_diff=0
cap_before=528 cap_after=528 cap_delta=0 nrg_delta=0 nrg_payoff=0

Change-Id: I7b5f7ccce56e93af7ea4e87d8e0ea6e2405f9c27
Signed-off-by: Ke Wang <ke.wang@spreadtrum.com>
(cherry picked from commit 0da783a605cd20d5a37c2a840e8a1fa641c09768)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>