In preparation for moving to atomic helpers, move the implicit sync
fence handling out of atomic commit and into the plane->prepare_fb()
hook. While we're at it, de-duplicate the mdp*_prepare_fb functions.

Changes in v4:
- Added
Reported-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>