msm: mdss: flush retire work before ctl_stop operation

The vsync_retire worker registers for vsync_handler and
removes it when client requests power_off. It may
possible that vsync_handler is removed from list
through two different contexts. One from worker thread
and other from ctl_stop_sub call. Such race condition
can lead to list corruption. Ideally, ov_off should
wait for retire worker flush before calling ctl_stop
to avoid such race condition.

Change-Id: I7d68d67d1fe1df07e568a5f40db745ce155d7d14
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>