sched/fair: dont run energy calculation unless necessary

The placement code calls find_best_target() to find the next cpu,
and if possible a backup cpu. After that it does an energy
evaluation between next, backup and prev cpu.

When find_best_target() finds the next or backup the same
as prev, we unnecessarily account for the same cpu twice
in energy evaluation.

So, drop the backup if its same as prev. If the next cpu
is same as the prev, make backup the next candidate.

Also replace task_cpu() with stack prev_cpu variable.

Change-Id: Ia2dc0c1b059ad515d99e7d66556795b24d30629c
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
[clingutla@codeaurora.org: Resolved merge conflicts, and include
 backup cpu check with target cpu.]
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>