drm/nouveau/disp: power down unused DP links during init

When booted with multiple displays attached, the EFI GOP driver on (at
least) Ampere, can leave DP links powered up that aren't being used to
display anything.  This confuses our tracking of SOR routing, with the
likely result being a failed modeset and display engine hang.

Fix this by (ab?)using the DisableLT IED script to power-down the link,
restoring HW to a state the driver expects.

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