sdm: Prioritize vsync register over commit

Back to back display commits can delay RegisterVsync()
-- Wait for previous retire fence.
   This ensures that driver is flushed.
-- Register for vsync
-- Start commit processing.

Change-Id: Id2838dc8fe54cf4af8032cc794e2a12c0f716b88
CRs-Fixed: 2583241
3 files changed