kernel: disable IGD (video DRM) support

kernel: disable IGD (video DRM) support

IGD is only useful when accelerating a VM guest that wants to direct
render to memory in the host's framebuffer, but since OpenWrt
typically runs on headless hardware, this serves no purpose.

Also build vfio with VFIO_NOIOMMU undefined (to get all of the code
enabled), but allow it to be enabled via boot-time modparams
settings (or at run-time via sysfs writes to
"/sys/module/vfio/parameters/enable_unsafe_noiommu_mode".

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>