drm/nouveau/dp: fix bad comparison in dp_link_train_commit()

The comparison (lpre == DP_TRAIN_PRE_EMPHASIS_9_5) is always false:
lpre is initialized as (lane & 0x0c) >> 2, which is at most 3, while
DP_TRAIN_PRE_EMPHASIS_9_5 is defined as (3 << 3).

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>