drm/nouveau/disp/dp: maintain link in response to hpd signal

This previously worked for the most part due to userspace doing a
modeset in response to HPD interrupts.  This will allow us to
properly handle cases where sync is lost for other reasons, or if
userspace isn't caring.

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

diff --cc drivers/gpu/drm/nouveau/core/engine/disp/dport.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/disp/dport.h
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/disp/nv50.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/disp/nvd0.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/disp/outpdp.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/disp/outpdp.h
Simple merge