drm/i915/selftests: Distinguish mock device from no wakeref

On systems that have no runtime-pm, we mark the wakeref as being -1. We
therefore cannot use that value for the mock-gt indicator, so opt for
-ENODEV instead. The wakeref should never be an error value -- one
hopes!

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/20190927211749.2181-2-chris@chris-wilson.co.uk