Just to be clear.
You installed Win8 Pro on your [unknown model] HP laptop, and you downloaded all the Windows 8 drivers from HP for your model laptop... especially the video drivers... and you installed them, and they all installed without an issue. After all that, the IGP doesn't work while in Windows?
Or, did you just download the video drivers for whatever the non-IGP chip is in your laptop, and installed those? Or, did you download both sets of drivers independently for both chips, install them both, and then expect it to switch based on demand?
Unfortunately, laptops that use dual video chipsets in this type of configuration have drivers made by the laptop manufacturer that are designed to work with the hardware as they have it set up in the laptop. The Windows 7 video drivers might work... and they might not. But if HP doesn't support Windows 8 on your model laptop, meaning if they don't have any Windows 8 drivers available for download, you might be out of luck as far as installing Windows 8 goes.
Hard to say without knowing the model laptop that you have. You know... hard to recommend what to download and all that.