UPSTREAM: powercap/dtpm_cpu: 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 0f71a6a0c74e1067839a239bcd7c855a5de675ad)
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Change-Id: Ic0a1888e5787a2d3dabe0a9ecd3d7497ea550250