intel: Add support for i945g to intel_stub_gpu.

I wanted to check the impact of nir-to-tgsi on the i915g driver, but I don't have an i915 set up. Similarly, for driconf refactoring I needed to make sure that classic i915_dri.so still loaded.

Note that I'm not too sure of the full impact of adding an i915 pci id to this core intel code -- we won't try to load any gen4+ stuff on this pci id due to modifying this code, will we?