How to show the only value in the Slicer Dropdown when RLS is applied

Hi, I have a Drop Down Slicer that has mutliple values of City. 

I need to provide my user the option of Select All as well as the Multi-select (that is, slicer can't be single select) 

My problem is design related. I have implemented RLS to my Report and using the RLS, some users will have access to only one city. 

However, they still see Select All option when they open the report (and if they use Dropdown of slicer, there's only one option of City according to their RLS. 

What I need to do is if the person opening the reports has access to only one city, then it shouldn't show "Select All" for him, but instead should show the City Name (because showing "Select All" has no purpose in this case) 

Is it somehow possible to do so? 

That is, if I view it with Beijing Role here, I still see the option for "Select All" while Beijing Role has access to no other city. 

Does anyone have any idea for this?

I can't change the Slicer to "Vertical List" because there are too many options and it has to be set at "Dropdown" 

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