disp: msm: sde: flush event thread workqueue before vm release

In some cases while transitioning between vm, vblank
work might be get executed on event thread after handoff is
completed on commit thread leading to crash. This change
flush the event thread queue during vm pre-release before
lending the io resources to the other vm.

Change-Id: I6a504fc2cec1c4985f7674bbca7fc5869bd8ad07
Signed-off-by: Yashwanth <yvulapu@codeaurora.org>