drm/i915: Align vswing sequences with old ddi buffer registers.

Vswing sequences on BXT and CNL are equivalent
to the ddi buffer registers setting on other platforms.

For some reason it got aligned with skl_ddi_set_iboost what
is semantically incorrect. This forced us to keep skipping
ddi buffer translation tables on the platforms that has
the vswing sequences.

v2: Don't mess with DP signal levels on this patch.

Cc: Vandana Kannan <vandana.kannan@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170829232230.23051-3-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_ddi.c

@@ -688,9 +688,6 @@ static void intel_prepare_dp_ddi_buffers(struct intel_encoder *encoder)
 	enum port port = intel_ddi_get_encoder_port(encoder);
 	const struct ddi_buf_trans *ddi_translations;
 
-	if (IS_GEN9_LP(dev_priv))
-		return;
-
 	switch (encoder->type) {
 	case INTEL_OUTPUT_EDP:
 		ddi_translations = intel_ddi_get_buf_trans_edp(dev_priv,
@@ -741,9 +738,6 @@ static void intel_prepare_hdmi_ddi_buffers(struct intel_encoder *encoder)
 	enum port port = intel_ddi_get_encoder_port(encoder);
 	const struct ddi_buf_trans *ddi_translations_hdmi;
 
-	if (IS_GEN9_LP(dev_priv))
-		return;
-
 	hdmi_level = intel_ddi_hdmi_level(dev_priv, port);
 
 	if (IS_GEN9_BC(dev_priv)) {
@@ -2154,7 +2148,9 @@ static void intel_ddi_pre_enable_dp(struct intel_encoder *encoder,
 
 	intel_display_power_get(dev_priv, dig_port->ddi_io_power_domain);
 
+	if (!IS_GEN9_LP(dev_priv) && !IS_CANNONLAKE(dev_priv))
 		intel_prepare_dp_ddi_buffers(encoder);
+
 	intel_ddi_init_dp_buf_reg(encoder);
 	intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_ON);
 	intel_dp_start_link_train(intel_dp);
@@ -2180,14 +2176,16 @@ static void intel_ddi_pre_enable_hdmi(struct intel_encoder *encoder,
 
 	intel_display_power_get(dev_priv, dig_port->ddi_io_power_domain);
 
-	intel_prepare_hdmi_ddi_buffers(encoder);
-	if (IS_GEN9_BC(dev_priv))
-		skl_ddi_set_iboost(encoder, level);
+	if (IS_CANNONLAKE(dev_priv))
+		cnl_ddi_vswing_sequence(encoder, level);
 	else if (IS_GEN9_LP(dev_priv))
 		bxt_ddi_vswing_sequence(dev_priv, level, port,
 					INTEL_OUTPUT_HDMI);
-	else if (IS_CANNONLAKE(dev_priv))
-		cnl_ddi_vswing_sequence(encoder, level);
+	else
+		intel_prepare_hdmi_ddi_buffers(encoder);
+
+	if (IS_GEN9_BC(dev_priv))
+		skl_ddi_set_iboost(encoder, level);
 
 	intel_dig_port->set_infoframes(&encoder->base,
 				       has_infoframe,