xwayland: Reset scheduled frames after hiding tablet cursor

Hiding the tablet tool cursor results in it being hidden forever after.
This is due to the stale frame callback that will neither be disposed
or replaced. This can be reproduced in krita (X11) as the pointer
cursor is hidden while over the canvas.

Clearing the frame callback ensures the correct behavior in future
xwl_tablet_tool_set_cursor() calls (i.e. a new cursor surface being
displayed, and a new frame callback created), and is 1:1
with xwl_seat_set_cursor() for pointers.

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
(cherry picked from commit dea4a746)