pointer: Fix the behavior of the pointer under DND

When grabbing with DND, we need to leave the pointer alone and
under the client's control. The code here was a bit messy before about
when it unset the window cursor -- it did it whenever there was no
current surface after repicking, which is a bit wrong, since it will
fire during a drag grab.

Move the check for this to update_cursor_surface, which is our standard
"sync" API for this, and then call update_cursor_surface after we set
the focus.