drm/i915/gt: Rename i915_gt_timelines

Since the anonymous i915_gt became struct intel_gt and encloses
struct i915_gt_timelines, rename i915_gt_timelines to intel_gt_timelines
to match its parentage.

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/20190621131640.28864-1-chris@chris-wilson.co.uk