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: Ifd63716ffe088e46832237b69ea7d7e1639ae1b7
CRs-fixed: 394139
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>
Signed-off-by: Arun Menon <menon@codeaurora.org>