BugLink: http://bugs.launchpad.net/bugs/1546572

The Rockchip driver cannot use drm_atomic_helper_wait_for_vblanks()
because it has hardware counters for neither vblanks nor scanlines.

In order to simplify re-implementing the functionality for this driver,
export the framebuffer_changed() helper so it can be reused.
Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
(cherry picked from commit c240906d)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>