disp: msm: sde: avoid extra vblank refcount from modeset

SDE encoder modeset add and remove the vblank refcount
based on userspace vblank refcount request. This logic
can have race condition with vblank update from
crtc event thread. It is already avoided with
Ibb810ec90e8 ("disp: msm: sde: manage vblank
refcount concurrency").

Change-Id: I1749bff2e2574500db03a405781eab7496f307b2
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>