drm/i915: Reject HDMI 12bpc if the sink doesn't indicate support

Check that the sink really declared 12bpc support before we enable it.
This should not actually never happen since it's mandatory for HDMI
sinks to support 12bpc if they support any deep color modes. But
reality disagrees with the theory and there are actually sinks in
the wild that violate the spec.

v2: Fix the output_types check
    Update commit message to state that these things are in fact real

Cc: stable@vger.kernel.org
Cc: Nicholas Sielicki <nicholas.sielicki@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99250
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213175818.24958-1-ville.syrjala@linux.intel.com
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
(cherry picked from commit c750bdd)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_hdmi.c

@@ -1298,16 +1298,34 @@ intel_hdmi_mode_valid(struct drm_connector *connector,
 
 static bool hdmi_12bpc_possible(struct intel_crtc_state *crtc_state)
 {
-	struct drm_device *dev = crtc_state->base.crtc->dev;
+	struct drm_i915_private *dev_priv =
+		to_i915(crtc_state->base.crtc->dev);
+	struct drm_atomic_state *state = crtc_state->base.state;
+	struct drm_connector_state *connector_state;
+	struct drm_connector *connector;
+	int i;
 
-	if (HAS_GMCH_DISPLAY(to_i915(dev)))
+	if (HAS_GMCH_DISPLAY(dev_priv))
 		return false;
 
 	/*
 	 * HDMI 12bpc affects the clocks, so it's only possible
 	 * when not cloning with other encoder types.
 	 */
-	return crtc_state->output_types == 1 << INTEL_OUTPUT_HDMI;
+	if (crtc_state->output_types != 1 << INTEL_OUTPUT_HDMI)
+		return false;
+
+	for_each_connector_in_state(state, connector, connector_state, i) {
+		const struct drm_display_info *info = &connector->display_info;
+
+		if (connector_state->crtc != crtc_state->base.crtc)
+			continue;
+
+		if ((info->edid_hdmi_dc_modes & DRM_EDID_HDMI_DC_36) == 0)
+			return false;
+	}
+
+	return true;
 }
 
 bool intel_hdmi_compute_config(struct intel_encoder *encoder,