BACKPORT: drm/fence: add drm_crtc_create_fence()

Instead of dealing with crtc details inside drm_atomic.c we should
just export a function that creates a new crtc fence for us and
use that.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1481046437-18778-1-git-send-email-gustavo@padovan.org


(cherry picked from commit 35f8cc3b9a92c661e5331d64fdfdedb518891d5c)
Signed-off-by: Marissa Wall <marissaw@google.com>

Change-Id: Ia127dc391cb0b6e8adb283a12009466f86e9df1b