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>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index a7049ea..fd72f3f 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -251,6 +251,10 @@
 			    &dev->mode_config.connector_list,	\
 			    base.head)
 
+#define for_each_digital_port(dev, digital_port)		\
+	list_for_each_entry(digital_port,			\
+			    &dev->mode_config.encoder_list,	\
+			    base.base.head)
 
 #define for_each_encoder_on_crtc(dev, __crtc, intel_encoder) \
 	list_for_each_entry((intel_encoder), &(dev)->mode_config.encoder_list, base.head) \