If these are functions, use function macros, to narrow down the matching scope.

Not sure if any of this is needed; I have come across things demanding something like this with not all types figured out (yet). Will this collide with anything?

typedef const char *    acpi_string;
union acpi_object {
        /* XXX FIXME */
        int type;
        struct {
                int                     value;
        } integer;
        struct {
                union acpi_object       *elements;
        } package;
};

Not sure if what is needed ?
ACPI_HANDLE is used in drm-kmod and is needed.
Not sure I understand what you mean.

The block of code I pasted in above... Did I just need that or do we inherit that from FreeBSD's ACPI implementation ... or would that break anything in yours?

I don't understand.
What's in this review is the only thing needed for drm, maybe you need more I don't know, this can be addressed later.

dokioki. I just wanted to make sure it's not going to break your stuff later.

I have no idea if it will break things or not, you can try yourself.
I'm going to commit this tomorow so it will be easier for you to test.