sched: EAS: update trg_cpu to backup_cpu if no energy saving for target_cpu

If no energy saving for target_cpu in the calculation of energy_diff(),
backup_cpu will be set as the new dst_cpu for the next calculation. At this
point, we also need update the new trg_cpu as backup_cpu to make sure the
subsequent calculation of energy_diff() is correct.

Change-Id: If3b35b6dc54865f1cb4b1603134102d4422227d5
Signed-off-by: Ke Wang <ke.wang@spreadtrum.com>
(cherry picked from commit b1923e22f4eca3e537e015d6ea3dce1187edea37)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>