clients/simple-dmabuf-feedback: use time instead of redraws

Weston uses a timeout of 2 seconds before it sends scanout
tranches to clients in order to not trigger excessive buffer
reallocations in clients.
`simple-dmabuf-feedback` in turn counts redraws (200) before
exiting. That doesn't work on e.g. 144Hz screens, thus use a
timer here as well.

Signed-off-by: Robert Mader <robert.mader@collabora.com>