sched/fair: use WALT signal for energy computation when WALT is enabled

Right now, irrespective of WALT enabled or not, energy computation uses
PELT signal which will be wrong when WALT is enabled. To make sure
we do proper calculations for WALT enabled case, use WALT signal.

Change-Id: Idcc4ffb8e5a6e7001a483ac4fa99e519ab6cfb67
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>