msm: cvp: Don't access deleted session during SSR

It may cause undefined behavior if CVP driver continue access
deleted session attributes during SSR (SubSystem Restart). The
change is to fix the potential issues.

Change-Id: I3f245eb966b10f0748f7f7be069c271d011fd01e
Signed-off-by: George Shen <sqiao@codeaurora.org>