UPSTREAM: drm/atomic-helper: Export framebuffer_changed()

UPSTREAM: drm/atomic-helper: Export framebuffer_changed()

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.

Change-Id: I80e2dc3b412d2299e6d97a9421e928dc32a9b63e
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
(cherry picked from commit c240906d36653944d5c049df7ce667a7e8bea6ac)