msm_fb: display: Wait for overlay done after DSI Video writeback switch

After writeback mode is switched for DSI Video interface, wait for
overlay done for prefill frame to avoid unnecessary fetches from previous
buffers resulting in page faults.

CRs-Fixed: 372389
Signed-off-by: Ravishangar Kalyanam <rkalya@codeaurora.org>
Change-Id: Iacd3775a718fe8e08404b4d119d447a88b628617