SiS 761 chipset does not support AGP cards but has AGP capability (for
the onboard video). At least PC Chips A31G board using this chipset has
an AGP-like AGPro slot that's wired to the PCI bus. Enabling AGP will
fail (GPU lockup and software fbcon, X11 hangs).

Add support for matching just the host bridge in nvkm_device_agp_quirks
and add entry for SiS 761 with mode 0 (AGP disabled).
Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>