drm/i915/selftests: Synchronize checking active status with retirement

If retirement is running on another thread, we may inspect the status of
the i915_active before its retirement callback is complete. As we expect
it to be running synchronously, we can wait for any callback to complete
by acquiring the i915_active.mutex.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022112111.9317-1-chris@chris-wilson.co.uk