sna: Skip undamaged TearFree redisplays

sna: Skip undamaged TearFree redisplays

If we have not had cause to flush the wait_for_shadow buffer during the
course of the rendering, then we never wrote to the backbuffer and its
contents are still identical to the current frontbuffer. So if the
wait_for_shadow is still flagged as required on the scanout, we know we
can safely discard the redisplay request.

References: https://bugs.freedesktop.org/show_bug.cgi?id=70905
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>