BACKPORT: drm/atomic: add drm_atomic_set_fence_for_plane()

This new function should be used by drivers when setting a implicit
fence for the plane. It abstracts the fact that the user might have
chosen explicit fencing instead.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1478513013-3221-1-git-send-email-gustavo@padovan.org


(cherry picked from commit 13b55664eec7b85607c4ab9d26a62b4af413a771)
Signed-off-by: Marissa Wall <marissaw@google.com>

Change-Id: If364103f7e85d915e13b84c0cb8ce37202c6f9b9