wayland: request frame callbacks on the topmost surface

Which is wl->video_surface for vo_dmabuf_wayland.

Listening on wl->surface results in freezes if it is occluded and
culled by the compositor. Which mutter does, and the wl_surface::frame
spec warns about:

> A server should avoid signaling the frame callbacks if the
> surface is not visible in any way, e.g. the surface is off-screen,
> or completely obscured by other opaque surfaces.