drm/nouveau/disp: fix dithering not being enabled on some eDP macbooks

commit a6a17859 upstream.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
[Maarten Lankhorst backported to 3.2,
 changing nv_connector->type to nv_connector->dcb->type]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/nouveau/nouveau_connector.c

@@ -616,7 +616,13 @@ nouveau_connector_detect_depth(struct drm_connector *connector)
 	if (nv_connector->edid && connector->display_info.bpc)
 		return;
 
-	/* if not, we're out of options unless we're LVDS, default to 8bpc */
+	/* EDID 1.4 is *supposed* to be supported on eDP, but, Apple... */
+	if (nv_connector->dcb->type == DCB_CONNECTOR_eDP) {
+		connector->display_info.bpc = 6;
+		return;
+	}
+
+	/* we're out of options unless we're LVDS, default to 8bpc */
 	if (nv_encoder->dcb->type != OUTPUT_LVDS) {
 		connector->display_info.bpc = 8;
 		return;