hw/pl061: Remove pointless comparison of array to null

Remove a pointless comparison of an array to null. (There is
no need to check whether s->out[i] is non-null as qemu_set_irq
will do that for us.) Spotted by Coverity (see bug 887883).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
