drm/i915: add i915_address_space_fini

We already have an i915_address_space_init, so for symmetry we should
also have a _fini, plus we already open code it twice. This then also
fixes a bug where we leak the timeline for the ggtt vm.

v2: don't forget about the struct_mutex for the ggtt path.

Fixes: 80b204bc ("drm/i915: Enable multiple timelines")
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/20161117210411.14044-1-chris@chris-wilson.co.uk