drm/i915/selftests: Check RPS controls

Check that the GPU does respond to our RPS frequency requests by setting
our desired frequency.

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/20200420172739.11620-3-chris@chris-wilson.co.uk