clients: Fix cursors when compositor gives wl_seat before wl_compositor

We have no guarantee that we can create a surface for the pointer at the
instant we receive a seat that will (probably eventually) need one.

Hold off until we receive an enter event before creating this - at that
point we know with certainty that wl_compositor is available, since we've
used it to create the surface that was entered.

Fixes #659

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>