drm/i915: Iterate through the initialized DDIs to prepare their buffers

Not every DDIs is necessarily connected can be strapped off and, in the
future, we'll have platforms with a different number of default DDI
ports. So, let's only call intel_prepare_ddi_buffers() on DDI ports that
are actually detected.

We also use the opportunity to give a struct intel_digital_port to
intel_prepare_ddi_buffers() as we'll need it in a following patch to
query if the port supports HMDI or not.

On my HSW machine this removes the initialization of a couple of
(unused) DDIs.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>