drm/i915/gt: Always call kref_init for the timeline

Always initialise the refcount, even for the embedded timelines inside
mock devices.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625233349.32371-2-chris@chris-wilson.co.uk