drm/vgem: Use lockless gem BO free callback

No dev->struct_mutex anywhere to be seen.

Cc: seanpaul@chromium.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464630800-30786-17-git-send-email-daniel.vetter@ffwll.ch