Modify drm_driver::device_is_agp to return a tri-state value to indicate

Modify drm_driver::device_is_agp to return a tri-state value to indicate
    that a device absolutely is, absolutely is not, or may or may not be
    AGP. Modify the i915 DRM to use this to force all i9x5 devices to be
    "AGP" (even the PCI-e devices).
Reported by: Lukas Hejtmanek