wayland/cursor-role: Increase buffer use count on construction

We may be assigned multiple times, if the surface is assigned to be a
cursor surface multiple times. Each time e.g. wl_pointer.set_cursor is
called, we'll be assigned.

While the role object exists, we'll handle buffer use count even when
we are not actively assigned, thus we should only handle the initial
assignment use count bump when constructing, so that we don't increase
it when reassigned, where the wl_resource may already have been
released.

https://bugzilla.gnome.org/show_bug.cgi?id=770402