drm/nouveau/dp: fix support for dpms

SOR_PWR has no effect to power-off DP links, unlike other SOR protocols.

Instead, on the source side, we cut power to the lanes after having put
the sink into D3.  Link training takes care of everything required to
bring it back again.

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

diff --cc drivers/gpu/drm/nouveau/nouveau_connector.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nv50_display.c
Simple merge