drm/i915/selftests: Correct frequency handling in RPS power measurement

Fix the frequency calculation by ensuring it uses the raw frequency only.
Update live_rps_power test to use the correct frequency values for logging
and comparison.

Signed-off-by: Sk Anirban <sk.anirban@intel.com>
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250113095912.356147-2-sk.anirban@intel.com