staging/android: do not let userspace trigger WARN_ON

Closing the timeline without waiting all fences to signal is not
a critical failure, it is just bad usage from userspace so avoid
calling WARN_ON in this case.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>