drm/i915/selftests: Do not try to sanitize mock HW

If we are mocking the device, skip trying to sanitize the pm HW state.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927210646.29664-1-chris@chris-wilson.co.uk