You can set a folder filter that will apply to all dashboards in that folder.

Firstly enter the folder you would like to apply a filter to, either through "Your Content" or using the left hand side panel.
​In this example we will be adding a folder filter to the "Business Overview" folder.

To create a folder filter click on the funnel icon in the header ribbon. This will open a the filter panel.

Select the dimension you want to filter by from the dropdown:

Then choose the value(s) you want to include or exclude. 

The available filter value will be presented in different ways depending on the number of values listed.

Once the values are selected, apply the filter by selecting the green tick and then publish those filters by selecting publish.