Some devices have the hardware to do certain things, but are then hamstrung by the software. Huawei can have a number of reasons as to why they did that with the stock camera application here. Whatever the reason is, the Huawei P9 Lite's camera supports the Camera2 API and this allows it to capture RAW images in a .DNG format. You can get this with 3rd-party apps, but you can also follow this guide laid out by XDA Senior Member zgomot if you want to enable it on the stock camera application.