ANDROID: sched/fair: Cap transient util in stune

boosted_cpu_util() sums the CFS and RT util signals before they are used
for frequency selection. While the util_avg signals are in sync,
util_est prevents cpu_util_cfs() from decreasing when a CFS task is
preempted by RT.

This util_est behaviour is beneficial in many scenarios, but it can
cause the sum of rt_util and cfs_util to go above SCHED_CAPACITY_SCALE.
Although benign, this transient value appears in the stune tracepoints,
and can cause confusion.

Work around the problem by capping the util sum to SCHED_CAPACITY_SCALE.

Bug: 120440300
Change-Id: I2be6eb157af86024e52ae11715f5637c77b201a3
Signed-off-by: Quentin Perret <quentin.perret@arm.com>