drm/nouveau/bios: log if auxch accesses fail, also return 0x00 from rd when it does

Logging at trace level, rather than as en error, as it seems conceivable
that failure could be normal under certain circumstances (new bios,
older sink that doesn't support a particular DPCD address)

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>