drm/i915/selftests: Prepare timeline tests for obj->mm.lock removal

We can no longer call intel_timeline_pin with a null argument,
so add a ww loop that locks the backing object.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-57-maarten.lankhorst@linux.intel.com