drm/i915: wire up SDVO hpd support on cpt/ppt

Now with Egbert Eich's hpd infrastructure rework merged this is dead
simple. And we need this to make output detection work on SDVO - with
the cleaned-up drm polling helpers outputs which claim to have hpd
support are no longer polled.

Now SDVO claims to do that, but it's not actually wired up. So just do
it.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>