drm/i915/guc/slpc: Add helper function slpc_measure_power

Previously, the RPS function was being used, which utilizes
raw frequency to calculate measured power. This commit introduces
a dedicated function specifically for measuring power in SLPC,
ensuring more accurate and reliable power measurements.

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/20250113095912.356147-3-sk.anirban@intel.com