drm/i915: Add some debug spam for intialising SDVO

During SDVO initialisation it would be useful to a have a record of the
individual devices we try to enable and later probe - in particular to
be able to see which fail.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>