drm/i915: don't leak global_timeline

We need to clean up the global_timeline in i915_gem_load_cleanup.

v2: don't forget about the struct_mutex, and also WARN_ON if we have any
remaining timelines before purging the global_timeline.

v3: it might be a good idea to first remove the global_timeline...duh!

Fixes: 73cb9701 ("drm/i915: Combine seqno + tracking into a global timeline struct")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1479415087-13216-1-git-send-email-matthew.auld@intel.com
Link: http://patchwork.freedesktop.org/patch/msgid/20161117210411.14044-2-chris@chris-wilson.co.uk