soc: hgsl: fix race of isync timeline when creating

In isync timeline create, after timeline object is attached into idr and
unlock the isync_timeline_lock, timeline object still is accessed. if
there is a release thread which try to release the same timeline object,
then maybe cause UAF issue.
Move the access operation into lock to avoid.

Change-Id: Ie2ff412b90924acb8f40e182f26c770b1f110a56
Signed-off-by: Kasin Li <quic_donglil@quicinc.com>