drm/i915/selftests: Skip energy consumption tests if not controlling freq

If we can not manipulate the frequency with RPS, then comparing min/max
power consumption is pointless / misleading. We will leave the warning
about not being able to control the frequency selection via RPS to other
tests so as not to confuse this more specialised check.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200420172739.11620-2-chris@chris-wilson.co.uk