hw/display/tc6393xb: limit irq handler index to TC6393XB_GPIOS

The ctz32() routine could return a value greater than
TC6393XB_GPIOS=16, because the device has 24 GPIO level
bits but we only implement 16 outgoing lines. This could
lead to an OOB array access. Mask 'level' to avoid it.

Reported-by: Moguofang <moguofang@huawei.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-id: 20171212041539.25700-1-ppandit@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
