ANDROID: sched/fair: Select correct capacity state for energy_diff

The util returned from group_max_util is not capped at the max util
present in the group, so it can be larger than the capacity stored in
the array. Ensure that when this happens, we always use the last entry
in the array to fetch energy from.

Tested with synthetics on Juno board.

Change-Id: I89fb52fb7e68fa3e682e308acc232596672d03f7
Signed-off-by: Chris Redpath <chris.redpath@arm.com>