drm/virtio: Use lockless gem BO free callback

No dev->struct_mutex anywhere to be seen.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464630800-30786-18-git-send-email-daniel.vetter@ffwll.ch