drm/amd/display: Retry when read dpcd caps failed.

Some DP panel not detected intermittently due to read dpcd
caps failed when doing hot plug.
[root cause] DC_HPD_CONNECT_INT_DELAY is set to 0, not delay
after HPD toggle and read dpcd data, while some panel need 4ms defer
to read.
[solution] Add a retry when read failed.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -2278,6 +2278,8 @@ static bool retrieve_link_cap(struct dc_link *link)
 	union edp_configuration_cap edp_config_cap;
 	union dp_downstream_port_present ds_port = { 0 };
 	enum dc_status status = DC_ERROR_UNEXPECTED;
+	uint32_t read_dpcd_retry_cnt = 3;
+	int i;
 
 	memset(dpcd_data, '\0', sizeof(dpcd_data));
 	memset(&down_strm_port_count,
@@ -2285,11 +2287,15 @@ static bool retrieve_link_cap(struct dc_link *link)
 	memset(&edp_config_cap, '\0',
 		sizeof(union edp_configuration_cap));
 
-	status = core_link_read_dpcd(
-			link,
-			DP_DPCD_REV,
-			dpcd_data,
-			sizeof(dpcd_data));
+	for (i = 0; i < read_dpcd_retry_cnt; i++) {
+		status = core_link_read_dpcd(
+				link,
+				DP_DPCD_REV,
+				dpcd_data,
+				sizeof(dpcd_data));
+		if (status == DC_OK)
+			break;
+	}
 
 	if (status != DC_OK) {
 		dm_error("%s: Read dpcd data failed.\n", __func__);