service/vi: Unstub GetDisplayService

This function is also supposed to check its given policy type with the
permission of the service itself. This implements the necessary
machinery to unstub these functions.

Policy::User seems to just be basic access (which is probably why vi:u
is restricted to that policy), while the other policy seems to be for
extended abilities regarding which displays can be managed and queried,
so this is assumed to be for a background compositor (which I've named,
appropriately, Policy::Compositor).