sna: Skip shadow redisplay if flips still pending

We shouldn't even be attempting to redisplay if there are flips pending,
so exit early and expect to be called again after the pending flips
complete. Exiting early avoids having to call sna_mode_wakeup() in what
used to be a potentially recursive manner (see commit af36a4ab
"sna: Defer submission of the next shadow frame until halfway through").

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>