disp: msm: sde: let dma-fence driver set timestamp on signal

Avoid corrupting the cb_list by setting timestamp before signal.
The fence driver changed in msm-5.4 by creating a union for the
fence cb_list, timestamp, and rcu. Therefore only one of these
fields can be used at a time. Fence driver will take care of
setting the timestamp during dma_fence_signal after it has
cached the cb_list. Also, use fence framework API to set fence
error instead of modifying the dma_fence struct directly.

Change-Id: I58dbaa495027cb575f4f0b03275ca4aa551ecacb
Signed-off-by: Steve Cohen <cohens@codeaurora.org>