drm/msm/sde: support fence release before fence signal

Fence create call returns the fd to caller client. Client
may close the fd before fence is signaled by timeline.
The close fd call is translated into fence_release followed
by fence memory free. If fence signal is called at later
point to adjust the timeline; it may try to pick the
dangle pointer fence from fence_list and try to delete
it. This patch fixes the race condition between fence
release and fence signal.

Change-Id: I28aaf625066b43fbb659d917adbf9dd577b4b7ea
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>