mdss: move to a kthread for vsync_retire_work_handler

vsync_retire_work_handler is in the critical display path and should
never be delayed because of other non-FIFO work.

bug 30115868
bug 62616267

Signed-off-by: Tim Murray <timmurray@google.com>
Change-Id: I3b60f64c0939efa81842f66aab3ef72441318c73