msm: wfd: Avoid exiting mdp task thread on error.

Sometimes during suspend resume, the mdp task thread
fails to dequeue buffer and errors out and exits.
Later when kthread_stop is called on mdp task, the
device crashes in kthread_stop because the mdp task
thread is invalid.

Change-Id: I42aee61b82c97c378c8497ea70eb53b3eeb24191
CRs-fixed: 394139
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>
Signed-off-by: Arun Menon <menon@codeaurora.org>
Signed-off-by: Neha Pandey <nehap@codeaurora.org>