FIXUP: sched/tune: fix payoff calculation for boost region

The definition of the acceptance regions as well as the translation of
these regions into a payoff value was both wrong which turned out in:
a) a wrong definition of payoff for the performance boost region
b) a correct "by chance" definition of the payoff for the performance
   constraint region (i.e. two sign errors together fixing the formula)

This patch provides a better description of the cut regions as well as
a fixed version of the payoff computations, which are now reduced to a
single formula usable for both cases.

Reported-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>