drm/nouveau/fifo/gk110: expose KeplerChannelGpfifoB

This class supports a WFI method (0x0078) that's not present on the
KeplerChannelGpfifoA class.

The binary driver exposes both classes on these GPUs for some reason,
though there doesn't appear to be any difference in the setup that's
done for each (ie. even if you allocate GpfifoA, the WFI method will
still work).

We shall just expose GpfifoB, as I don't see a good reason to report
the presence of both.

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