ACPI: video: Ignore ACPI video devices that aren't present in hardware

Vendors often ship machines with a choice of integrated or discrete
graphics, and use the same DSDT for both. As a result, the ACPI video
module will locate devices that may not exist on this specific platform.

Attempt to determine whether the device exists or not, and abort the
device creation if it do not exist.

Signed-off-by: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: Len Brown <len.brown@intel.com>