vulkan: handle push DUT with emulated secondaries
We need some manual logic to work out the size of pData, so we handroll this one. This fixes push DUT with emulated secondaries.
Affects dEQP-VK.binding_model.shader_access.secondary_cmd_buf.pushtempl* if emulated secondaries are used.
Neither panvk nor dozen support push DUT yet, so this isn't hurting anyone and doesn't need to be cc'd stable. But hopefully panvk & dozen get on that :}