drm/nouveau/drm/nouveau: Don't forget to label dp_aux devices

This makes debugging with DP tracing a lot harder to interpret, so name
each i2c based off the name of the encoder that it's for

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Karol Herbst <karolherbst@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>