x86: Allow disabling IGD on Intel Queensbay

Add a Kconfig option to disable the Integrated Graphics Device (IGD)
so that it does not show in the PCI configuration space as a VGA
disaplay controller. This gives a chance for U-Boot to run PCI/PCIe
based graphics card's VGA BIOS and use that for the graphics console.

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