drm/i915/guc/slpc: Add SLPC selftest live_slpc_power

A fundamental assumption is that at lower frequencies,
not only do we run slower, but we save power compared to
higher frequencies.
live_slpc_power checks if running at low frequency saves power

v2: re-use code to measure power
    fixed cosmetic review comments (Vinay)

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/20220923110043.789178-4-riana.tauro@intel.com