Brandish hook: read framebuffer index offset from instruction stream, drop memcpy()

Read the offset for the framebuffer index from the instruction stream instead of using a literal constant, so the hook works for other games using the same function but a different offset (e.g. Zero no Kiseki). Also remove the memcpy() since the original function will do it anyway (and may do it differently if other games call the function with different arguments).
Good call, forgot about that. In this case it's an offset from a base pointer, so I imagine that if it ever reached 0x8000 there would be an extra lis/addu or some such inserted and the fb_index_load instruction check would fail, so it shouldn't matter. But I'll fix it anyway just for correctness.