drm/i915/selftests: Split RPS frequency measurement

Split the frequency measurement into two modes, so that we can judge the
impact of the llc setup on top of the pure CS frequency scaling.

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/20200420172739.11620-4-chris@chris-wilson.co.uk