Create a stored procedure. Run it in all companies.
Open SSRS Report Builder.
Create a normal valid data source, because the one that we're going to use will not 'test', and can't be used to validate SQL objects.
Then, create a Data Source called 'Company'.
'Build' it first, and then go into the expression builder and edit to as shown.
Create the Dataset as shown. Use the valid Data Source for now, we'll change it later.
Insert a Matrix, follow the wizard. Format the report
When you're ready, go into the Dataset and change the Data Source back to 'Company'
That's it! Give it a try, let me know if I left out any steps or didn't explain clearly enough.