Following expression can be used to show full AX company name from legal entities in SSRS report.
=Microsoft.Dynamics.Framework.Reports.DataMethodUtility.GetFullCompanyNameForUser(Parameters!AX_CompanyName.Value, Parameters!AX_UserContext.Value)
=Microsoft.Dynamics.Framework.Reports.DataMethodUtility.GetFullCompanyNameForUser(Parameters!AX_CompanyName.Value, Parameters!AX_UserContext.Value)