cpuidle: governors: teo: Decay metrics below DECAY_SHIFT threshold

cpuidle: governors: teo: Decay metrics below DECAY_SHIFT threshold

If a given governor metric falls below a certain value (8 for
DECAY_SHIFT equal to 3), it will not decay any more due to the
simplistic decay implementation.  This may in some cases lead to
subtle inconsistencies in the governor behavior, so change the
decay implementation to take it into account and set the metric
at hand to 0 in that case.

Suggested-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Tested-by: Christian Loehle <christian.loehle@arm.com>
Link: https://patch.msgid.link/2819353.mvXUDI8C0e@rafael.j.wysocki