drm/msm: Rename r/w_fence in GEM to _timestamp

It seems timestamp is better than fence in GEM object.
And in future the user generated timestamp will store here.

Change-Id: I0dba02c94c82658a80bf20bb5667329634cfa245
Signed-off-by: Yajun Li <yajunl@codeaurora.org>
Signed-off-by: Kasin Li <donglil@codeaurora.org>