drm/i915/dp: there is no audio on port A

commit 9fcb1704 upstream.

The eDP port A register on PCH split platforms has a slightly different
register layout from the other ports, with bit 6 being either alternate
scrambler reset or reserved, depending on the generation. Our
misinterpretation of the bit as audio has lead to warning.

Fix this by not enabling audio on port A, since none of our platforms
support audio on port A anyway.

v2: DDI doesn't have audio on port A either (Sivakumar Thulasimani)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89958Reported-and-tested-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/gpu/drm/i915/intel_dp.c

@@ -852,7 +852,7 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 		pipe_config->has_pch_encoder = true;
 
 	pipe_config->has_dp_encoder = true;
-	pipe_config->has_audio = intel_dp->has_audio;
+	pipe_config->has_audio = intel_dp->has_audio && port != PORT_A;
 
 	if (is_edp(intel_dp) && intel_connector->panel.fixed_mode) {
 		intel_fixed_panel_mode(intel_connector->panel.fixed_mode,
@@ -1578,8 +1578,8 @@ static void intel_dp_get_config(struct intel_encoder *encoder,
 	int dotclock;
 
 	tmp = I915_READ(intel_dp->output_reg);
-	if (tmp & DP_AUDIO_OUTPUT_ENABLE)
-		pipe_config->has_audio = true;
+
+	pipe_config->has_audio = tmp & DP_AUDIO_OUTPUT_ENABLE && port != PORT_A;
 
 	if ((port == PORT_A) || !HAS_PCH_CPT(dev)) {
 		if (tmp & DP_SYNC_HS_HIGH)