drm/msm: use kthread_create_worker instead of kthread_run

Use kthread_create_worker to simplify the code and optimise
the manager struct: msm_drm_thread. With this change, we
could remove struct element (struct task_struct *thread &
struct kthread_worker worker), instead, use one point (struct
kthread_worker *worker).

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>