drm/amd/display: Handle branch device with DFP count = 0 case.

[Why]
When you have a SST branch device the driver, Even no sink device connected,
it also send HPD with a valid EDID. Driver will config it to DP sink.
Therefore, there're two displays in display setting.

DPCD 0x05, DFP_PRESENT = 1 (branch device),
DFP_TYPE = 00 (Display Port)

[How]
Driver determine DPCD 0x05 DFP_PRESENT = 1(branch) as an active dongle
And check DFP count.
Signed-off-by: Hugo Hu <hugo.hu@amd.com>
Reviewed-by: Hugo Hu <Hugo.Hu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -720,9 +720,8 @@ bool dc_link_detect(struct dc_link *link, enum dc_detect_reason reason)
 					same_dpcd = false;
 			}
 			/* Active dongle plug in without display or downstream unplug*/
-			if (link->type == dc_connection_active_dongle
-					&& link->dpcd_caps.sink_count.
-					bits.SINK_COUNT == 0) {
+			if (link->type == dc_connection_active_dongle &&
+				link->dpcd_caps.sink_count.bits.SINK_COUNT == 0) {
 				if (prev_sink != NULL) {
 					/* Downstream unplug */
 					dc_sink_release(prev_sink);

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -2226,11 +2226,7 @@ bool is_mst_supported(struct dc_link *link)
 
 bool is_dp_active_dongle(const struct dc_link *link)
 {
-	enum display_dongle_type dongle_type = link->dpcd_caps.dongle_type;
-
-	return (dongle_type == DISPLAY_DONGLE_DP_VGA_CONVERTER) ||
-			(dongle_type == DISPLAY_DONGLE_DP_DVI_CONVERTER) ||
-			(dongle_type == DISPLAY_DONGLE_DP_HDMI_CONVERTER);
+	return link->dpcd_caps.is_branch_dev;
 }
 
 static int translate_dpcd_max_bpc(enum dpcd_downstream_port_max_bpc bpc)
@@ -2264,6 +2260,9 @@ static void get_active_converter_info(
 		return;
 	}
 
+	/* DPCD 0x5 bit 0 = 1, it indicate it's branch device */
+	link->dpcd_caps.is_branch_dev = ds_port.fields.PORT_PRESENT;
+
 	switch (ds_port.fields.PORT_TYPE) {
 	case DOWNSTREAM_VGA:
 		link->dpcd_caps.dongle_type = DISPLAY_DONGLE_DP_VGA_CONVERTER;

drivers/gpu/drm/amd/display/dc/dc.h

@@ -689,6 +689,8 @@ struct dpcd_caps {
 
 	/* dongle type (DP converter, CV smart dongle) */
 	enum display_dongle_type dongle_type;
+	/* branch device or sink device */
+	bool is_branch_dev;
 	/* Dongle's downstream count. */
 	union sink_count sink_count;
 	/* If dongle_type == DISPLAY_DONGLE_DP_HDMI_CONVERTER,