drm/msm/sde: enable output_fence property

Add an 'output_fence' property to the crtc to support both
Linux output fence semantics and Android release fences.

A 'prepare_fence' callback is added to msm_atomic_commit() after
swapping the incoming atomic state but before spawning any
worker threads to process the commit. This guarantees that the
correct sync point value may be determined by the driver at any
point after the relevant commit request.

Conflicts:
	drivers/gpu/drm/msm/msm_atomic.c

Change-Id: I7c024709f052fbf49f1c9e438d10a3c9f9aab023
Signed-off-by: Clarence Ip <cip@codeaurora.org>
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>
[narendram@codeaurora.org: resolve msm_atomic.c conflict]
Signed-off-by: Narendra Muppalla <narendram@codeaurora.org>