drm/i915/selftests: Hold task reference to reset worker

As the worker may exit by itself, we need to hold a task reference to it
in the parent.

References: https://bugs.freedesktop.org/show_bug.cgi?id=108735


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181120120601.24083-1-chris@chris-wilson.co.uk