drm/i915/selftests: Move gpu energy measurement into its own little lib

Move the handy utility to measure the GPU energy consumption using RAPL
msr into a common lib so that it can be reused easily.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200417152018.13079-1-chris@chris-wilson.co.uk