UPSTREAM: powercap/dtpm_devfreq: Use new Energy Model interface to get table

Energy Model framework support modifications at runtime of the power
values. Use the new EM table API which is protected with RCU. Align the
code so that this RCU read section is short.

This change is not expected to alter the general functionality.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

(cherry picked from commit 9c49bc4e005af2f54249b604e1a7af85d0d1cdff)
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Change-Id: Iea70e2d9d360f181a8558ce93c9ec5ecae1866b7