drm/i915: add i915_address_space_fini

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: 80b204bce8f2 ("drm/i915: Enable multiple timelines")
Cc: Chris Wilson <[email protected]>
Signed-off-by: Matthew Auld <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]