drm/i915: Remove duplicate DDI enabling logic from MST path

The logic to enable a DDI in intel_mst_pre_enable_dp() is essentially
the same as in intel_ddi_pre_enable_dp(). So reuse the latter function
by calling the post_disable hook on the intel_dig_port instead of
duplicating that code.

v2: Don't oops because of a NULL encoder->crtc. (Ville)
v3: Warn for MST + PORT_E too. (Ville)
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302125857.14665-8-ander.conselvan.de.oliveira@intel.com

drivers/gpu/drm/i915/intel_ddi.c

@@ -1713,6 +1713,8 @@ static void intel_ddi_pre_enable_dp(struct intel_encoder *encoder,
 	enum port port = intel_ddi_get_encoder_port(encoder);
 	struct intel_digital_port *dig_port = enc_to_dig_port(&encoder->base);
 
+	WARN_ON(link_mst && (port == PORT_A || port == PORT_E));
+
 	intel_dp_set_link_params(intel_dp, link_rate, lane_count,
 				 link_mst);
 	if (encoder->type == INTEL_OUTPUT_EDP)

drivers/gpu/drm/i915/intel_dp_mst.c

@@ -159,26 +159,9 @@ static void intel_mst_pre_enable_dp(struct intel_encoder *encoder,
 
 	DRM_DEBUG_KMS("%d\n", intel_dp->active_mst_links);
 
-	if (intel_dp->active_mst_links == 0) {
-		intel_ddi_clk_select(&intel_dig_port->base,
-				     pipe_config->shared_dpll);
-
-		intel_display_power_get(dev_priv,
-					intel_dig_port->ddi_io_power_domain);
-
-		intel_prepare_dp_ddi_buffers(&intel_dig_port->base);
-		intel_dp_set_link_params(intel_dp,
-					 pipe_config->port_clock,
-					 pipe_config->lane_count,
-					 true);
-
-		intel_ddi_init_dp_buf_reg(&intel_dig_port->base);
-
-		intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_ON);
-
-		intel_dp_start_link_train(intel_dp);
-		intel_dp_stop_link_train(intel_dp);
-	}
+	if (intel_dp->active_mst_links == 0)
+		intel_dig_port->base.pre_enable(&intel_dig_port->base,
+						pipe_config, NULL);
 
 	ret = drm_dp_mst_allocate_vcpi(&intel_dp->mst_mgr,
 				       connector->port,