[ENERGY] Energy wasn't computed correctly when pstates are changed

[ENERGY] Energy wasn't computed correctly when pstates are changed

Time 0-100: Pstate 0 was used
Time 100:   Pstate 1 is now used

The energy plugin used Pstate 1 at point 100 to compute the energy
consumed from time 0 to time 100. Of course, pstate 0 needs to be used
but the plugin had no way to keep track of that.

This patch fixes that