The problem with GMA500 is that it also implements the IGD Opregion spec, and the ACPI video driver detects this and refuses to bind. But the GMA500 kernel driver doesn't implement support for the spec and so doesn't call the function that triggers the ACPI video registration. Working around this is simple - just add acpi_video_register() to the init function of the GMA500 drm. But note that this means that you're failing to implement the spec properly, and there's potential for stuff to be broken. A full implementation of the spec for GMA500 wouldn't be especially difficult, but there's no docs and I have no hardware so I'm not going to do it myself.
The reason I bring this up is that various people have been approaching this problem in a different way. It's easy to assume that the check in the acpi driver was naively assuming that all Intel hardware was driven by i915 and that this patch was broken. It's actually entirely correct and the (out of tree) GMA500 driver was broken. If Intel had made the effort to get their code properly upstream, it'd have been fixed there when the original change was made and nobody would ever have had a problem. Just say no to out of tree drivers.
