drm/amdgpu/dm: Fix NULL pointer crash during DP MST hotplug

This patch checks the return value of the function
dc_link_add_remote_sink before using it. This was causing
a crash during consecutive hotplugs of DP MST displays.

Cc: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c

@@ -229,6 +229,11 @@ static int dm_dp_mst_get_modes(struct drm_connector *connector)
 			(aconnector->edid->extensions + 1) * EDID_LENGTH,
 			&init_params);
 
+		if (!dc_sink) {
+			DRM_ERROR("Unable to add a remote sink\n");
+			return 0;
+		}
+
 		dc_sink->priv = aconnector;
 		/* dc_link_add_remote_sink returns a new reference */
 		aconnector->dc_sink = dc_sink;