pointer: Don't fizzle out surface changes too soon

During a DND grab, pointer->focus_surface is NULL, since the wl_pointer
doesn't have any focused surface (it's in drag mode). In this case, the
drag interface has control of the focus, and when dragging into a NULL
surface, drag_grab_focus won't get called, properly detaching it from
the previous surface.

Let the interface->focus implementation do the fizzling out.

In the future, we should split out wl_pointer's implementation
(pointer->focus_surface) from the Wayland side of the generic pointer
wrapper (pointer->current) and use our event routing system to determine
or similar whether it should go to wl_pointer or wl_data_device.