msm: kgsl: Signal dump_gate only when worker finishes work

Currently we are accessing snapshot structure after signaling
dump_gate. In corner cases this worker can get preempted
before driver reads this variable, and eventually client
dumps the snapshot and snapshot object gets released.
When worker gets back in execution it will read freed memory
and will case kernel panic.

To fix this race, do completion for dump_gate after finishing
work in worker kgsl_snapshot_save_frozen_objs.

Change-Id: Iaf3c9f9e75a6e3e19653d2d166e2da624796ec98
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>