drm/i915/dsi: Do not read the transcoder register.

As per the Bspec, port mapping is fixed for mipi dsi.

v2: Reuse the existing function (Jani)
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119072004.4093-1-vandita.kulkarni@intel.com

drivers/gpu/drm/i915/display/intel_display.c

@@ -10577,16 +10577,21 @@ static void haswell_get_ddi_port_state(struct intel_crtc *crtc,
 				       struct intel_crtc_state *pipe_config)
 {
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
+	enum transcoder cpu_transcoder = pipe_config->cpu_transcoder;
 	struct intel_shared_dpll *pll;
 	enum port port;
 	u32 tmp;
 
-	tmp = I915_READ(TRANS_DDI_FUNC_CTL(pipe_config->cpu_transcoder));
-
+	if (transcoder_is_dsi(cpu_transcoder)) {
+		port = (cpu_transcoder == TRANSCODER_DSI_A) ?
+						PORT_A : PORT_B;
+	} else {
+		tmp = I915_READ(TRANS_DDI_FUNC_CTL(cpu_transcoder));
 		if (INTEL_GEN(dev_priv) >= 12)
 			port = TGL_TRANS_DDI_FUNC_CTL_VAL_TO_PORT(tmp);
 		else
 			port = TRANS_DDI_FUNC_CTL_VAL_TO_PORT(tmp);
+	}
 
 	if (INTEL_GEN(dev_priv) >= 11)
 		icelake_get_ddi_pll(dev_priv, port, pipe_config);