disp: msm: sde: manage vblank refcount concurrency

Vblank refcount can reach out of sync with below case
 1. event_thread triggers the vblank_enable
 2. commit_thread triggers the modeset
   2.a modeset resets the vblank refcount with mode_set
 3. event_thread triggers the vblank_disable

Event 2.a resets the vblank refcount and vblank disable
request after 2.a is going to fail. This can be fixed
by avoiding concurrency between mode_set call and vblank
request.

Change-Id: Ibb810ec90e81d63feee443f1c37dd736d5cfac0d
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>