drm/i915/selftests: Add helper function measure_power

move the power measurement and the triangle filter
to a different function. No functional changes.

Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220923110043.789178-3-riana.tauro@intel.com