drm/i915/selftests: Teach timelines to take intel_gt as its argument

The timelines selftests are [mostly] hardware centric and so want to use
the gt as its target.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191016113840.1106-1-chris@chris-wilson.co.uk