I have been in a situation like this before. I think this depends from company to company but I think that if you're using a laptop for your office job, then your company should provide it. But if the company you are with is a startup, I guess you'll have to understand that resources are limited as they may require you to use your own laptop. For an upgrade, usually the company will allow it because it does affect your work isn't it?

I believe whatever helps you do your job is part of your job. So bring your own desktop if you have to, after all as a designer the good works you produce end up as your own portfolio anyway