drm-howto: wait for pending page-flips to complete during exit

If we perform an immediate modeset via drmModeSetCrtc(), we should go sure
that there is no pending page-flip. This isn't strictly neccessary, but
some drivers are buggy if we do that so just be safe here.

Reported-by: Mark Zhang
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>