PM / EM: Micro optimization in em_pd_energy

If the sum of the utilization of CPUs in a power domain is zero,
return the energy as 0 instead of doing any math.

Change-Id: I9b1a83d210c30a8a86da26f94ac0c2f855d2ed10
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>