msm: kgsl: Requeue context if dispatcher signaled to stop

Correctly requeue a context on dispatcher->pending list if a cmdbatch
didn't submit because the dispatcher signaled to stop submitting
commands.

CRs-Fixed: 720249
Change-Id: I553e9db139978b6b269255a643087363c28e04bc
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>