x86: crownbay: Use external graphics card by default

The board routes the Integrated Graphics Device (IGD) to an LVDS
panel, which is less popular than a PCIe based graphics card.

Disable the IGD so that it does not show up in the PCI configuration
space as a VGA display controller, so we can use an external PCIe
graphics card with whatever cable we have.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>