disp: msm: sde: avoid vblank notification for cwb

The vsync callback for concurrent writeback is
not necessary. This would conflict with vblank
notification of primary as both belongs to the
same crtc.

Change-Id: Idb67915de086f94feb231d61b6f7e4e068a1ac35
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>