In the magit-status buffer I can press d-wd on a file to open a magit-diff buffer with the diff of that specific file without whitespace changes. When I go back to magit-status though, the diffs there still show the original whitespace changes. I'm not sure if this is built into magit already but what I'd like is for the original magit-status buffer to ignore whitespace changes as well (being able to toggle this would be amazing)
In magit-status, enter the diff mode by pressing d like you've done. Make your -w change. Then press C-x to see that there is an extended magit menu with more available items. You'll then see that C-s is a means to Save the settings you've made (e.g., --ignore-all-space) for future sessions.