drm/nouveau/nouveau: Disable AGP for SiS 761

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>