Expose if GPU is discrete or not

To allow consumers to make a more educated decision about what GPU to choose Switcheroo-control could expose if a GPU is discrete or not.

There are multiple ways to achieve this, the best would be checking if the GPU has dedicated vram (this is how Vulkan does it), but it appears that there is no sysfs interface for getting the memory.

Applications like nvtop have different code paths for different GPUs, with the Nvidia code path dynamically loading the NVML library.

Alternatively the PCI interface could be parsed since, statistically speaking, the iGPU tends to be on Bus 0.

Other heuristics like always considering Nvidia GPUs to be discrete, since Nvidia does not produce any integrated GPUs, could also be added.