drm/amd/display: check dpcd read status of DP_DOWNSTREAM_PORT_0

[Why]
For special DP convert VGA dongle, core_link_read_dpcd for
DP_DOWNSTREAM_PORT_0 always return fail, then dongle_type will be set to
DISPLAY_DONGLE_NONE, which will cause desktop resolution abnormal.

[How]
If dpcd for DP_DOWNSTREAM_PORT_0 read fail, skip set dongle_type =
DISPLAY_DONGLE_NONE.
Signed-off-by: jinlong zhang <jinlong.zhang@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -3097,8 +3097,8 @@ static void get_active_converter_info(
 		uint8_t det_caps[16]; /* CTS 4.2.2.7 expects source to read Detailed Capabilities Info : 00080h-0008F.*/
 		union dwnstream_port_caps_byte0 *port_caps =
 			(union dwnstream_port_caps_byte0 *)det_caps;
-		core_link_read_dpcd(link, DP_DOWNSTREAM_PORT_0,
-				det_caps, sizeof(det_caps));
+		if (core_link_read_dpcd(link, DP_DOWNSTREAM_PORT_0,
+				det_caps, sizeof(det_caps)) == DC_OK) {
 
 			switch (port_caps->bits.DWN_STRM_PORTX_TYPE) {
 			/*Handle DP case as DONGLE_NONE*/
@@ -3155,6 +3155,7 @@ static void get_active_converter_info(
 				break;
 			}
 		}
+	}
 
 	ddc_service_set_dongle_type(link->ddc, link->dpcd_caps.dongle_type);