sched/fair: honor uclamp restrictions in fbt()

While calculating untilization of CPU during task placement in fbt(),
current code doesn't take uclamp into account which would lead to
selection of incorrect CPU for the task when uclamp restrictions
are in place for the task.

Change-Id: I8371affe3b37733d222e5c57953e53f91fc19a53
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>