msm: kgsl: Simplify timelineobj cleanup

Use the same path for timelineobj retire and destroy. This keeps the
timelineobj valid until the preceding cmdbatches retire, and the scheduler
retires it during normal operation or context detach. This simplifies
cleanup when userspace detaches a context with timelineobjs in flight.

Change-Id: I8812acd045ee13bf965fea1361cf867baf7345a0
Signed-off-by: Lynus Vaz <quic_lvaz@quicinc.com>
(cherry picked from commit 65d32f99)