sched/fair: fix array out of bounds access in select_energy_cpu_idx()

We are using an incorrect index while initializing the nrg_delta for
the previous CPU in select_energy_cpu_idx(). This initialization
it self is not needed as the nrg_delta for the previous CPU is already
initialized to 0 while preparing the energ_env struct.

Change-Id: Iee4e2c62f904050d2680a0a1df646d4d515c62cc
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>