drm/i915: Don't write the HDMI buffer translation entry when not needed

We don't actually need to write the HDMI entry on DDIs that have no
chance to be used as HDMI ports.

While this patch shouldn't change the current behaviour, it makes
further enabling work easier as we'll have an eDP table filling the full
10 entries.

v2: Rely on the logic from intel_ddi_init() to figure out if the DDI port
    supports HDMI or not (Paulo).
Suggested-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
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>

drivers/gpu/drm/i915/intel_ddi.c

@@ -191,6 +191,12 @@ enum port intel_ddi_get_encoder_port(struct intel_encoder *intel_encoder)
 	}
 }
 
+static bool
+intel_dig_port_supports_hdmi(const struct intel_digital_port *intel_dig_port)
+{
+	return intel_dig_port->hdmi.hdmi_reg;
+}
+
 /*
  * Starting with Haswell, DDI port buffers must be programmed with correct
  * values in advance. The buffer values are different for FDI and DP modes,
@@ -294,6 +300,9 @@ static void intel_prepare_ddi_buffers(struct drm_device *dev,
 		reg += 4;
 	}
 
+	if (!intel_dig_port_supports_hdmi(intel_dig_port))
+		return;
+
 	/* Choose a good default if VBT is badly populated */
 	if (hdmi_level == HDMI_LEVEL_SHIFT_UNKNOWN ||
 	    hdmi_level >= n_hdmi_entries)