iris: refcount separate screen objects for resource tracking
this screen object can never, ever be accessed like this in a resource, as it may have previously been replaced by a wrapper (e.g., driver trace) which will then explode when it is accessed directly
instead, keep a separate screen ref on the resource which is known to be the actual driver object and not a wrapper
Fixes: 0a497eb1 ("iris: make resources take a ref on the screen object")