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

(cherry picked from commit 91fd2c322af5c111309e343e7d58f6e5e1841823)

CRs-Fixed: 1081525
Change-Id: Ic97744a81198ff303f9c87e1cf7c2468b4356a25
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>