drm/nouveau/disp/sor/gf119: both links use the same training register

It appears that, for whatever reason, both link A and B use the same
register to control the training pattern.  It's a little odd, as the
GPUs before this (Tesla/Fermi1) have per-link registers, as do newer
GPUs (Maxwell).

Fixes the third DP output on NVS 510 (GK107).

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org