drm/i915/gt: Track timeline activeness in enter/exit

Lift moving the timeline to/from the active_list on enter/exit in order
to shorten the active tracking span in comparison to the existing
pin/unpin.

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/20190815205709.24285-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_pm.c

@@ -37,7 +37,6 @@ static void i915_gem_park(struct drm_i915_private *i915)
 	for_each_engine(engine, i915, id)
 		call_idle_barriers(engine); /* cleanup after wedging */
 
-	intel_timelines_park(i915);
 	i915_vma_parked(i915);
 
 	i915_globals_park();