Add an helper to wait for all page flips of an atomic state to be done.

v2:
- Pimp kerneldoc as discussed with Boris on irc
- Add missing doc for @dev.
- Use old_state for consitency with wait_for_vblanks

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com> (v1)
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1496392332-8722-2-git-send-email-boris.brezillon@free-electrons.com