drm/nouveau/kms/nv50: ensure encoder normal power state is enabled at startup

drm/nouveau/kms/nv50: ensure encoder normal power state is enabled at startup

To handle low-power DPMS states, we currently change an OR's (Output
Resource) normal (active) power state to be off, leaving the rest of
the display configured as usual.

Under atomic modesetting, we will instead be doing a full modeset to
tear down the pipe fully when entering a low-power state.

As we'll no longer be touching the OR's PWR registers during runtime
operation, we need to ensure the normal power state is set correctly
during initialisation.

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