This should be impossible due to the wait for outstanding flips that the
caller is meant to perform prior to updating the scanout base. Paranoia
tells me to check anyway.

References: https://bugs.freedesktop.org/show_bug.cgi?id=75502Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>