drm/nouveau/disp/dp: add support for lttprs

Add support for Link-Training Tunable PHY Repeaters, required to support
higher bit rates on longer cables, as well as USB-C on certain docks and
laptops.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://gitlab.freedesktop.org/drm/nouveau/-/merge_requests/17