backend/drm: expose vendor product code

This is a tiny change that exposes the vendor product code.

For some monitors (at least mine) that have different modes, this seems to be the only "identifying" field in the EDID information that changes between those modes. The only other thing I could find that changes is the availability of more refresh rates and HDR stuff, but I would consider it a workaround of sorts to match configuration to a monitor based on those points, at least for my use cases. Exposing this field makes it possible to configure this kind of monitor per mode more reliably.