drm/nouveau/disp/dp: only check for re-train when the link is active

An upcoming commit will limit link training to only when the sink is
meant to be displaying an image.

We still need IRQs enabled even when the link isn't trained (for MST
messages), but don't want to train the link unnecessarily.

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