drm/i915/selftests: Add delay to stabilize frequency in live_rps_power

Add delays to allow frequency stabilization before power measurement
to fix sporadic power conservation issues in live_rps_power test.

v2:
  - Move delay to respective function (Badal)

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://patchwork.freedesktop.org/patch/msgid/20241203061114.2790448-1-sk.anirban@intel.com