clients: Set the hotspot with attach if we already have a valid cursor

We want atomic hotspot updates - this can't happen with
wl_pointer_set_cursor. So if we have a surface that already has a cursor
role, just update the hotspot when attaching new content.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>
(cherry picked from commit 646cc1b3)