drm/i915/selftests: Refactor RC6 power measurement and error handling

Revise the power measurement logic to save and evaluate energy values.
Previously, the test only checked whether the system had entered the RC6
state, without considering any potential interruptions in that state.
This update introduces a threshold check to ensure that the GPU remains
in the RC6 state properly during the specified sleep duration.

v3:
  - Reorder threshold check (Badal)

v4:
  - Improved commit message (Anshuman)

v5:
  - Rename variables for improved readability (Anshuman)

Signed-off-by: Sk Anirban <sk.anirban@intel.com>
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://lore.kernel.org/r/20250327191924.4131598-1-sk.anirban@intel.com