msm: mdss: Add commit handler to writeback display

Add support for a "vsync handler" for writeback displays to be able to
schedule tasks to be run as soon as the frame has completed.

Change-Id: I8c6945a6e914e307de67b2ec6df25f4509a50da7
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>
Signed-off-by: Ping Li <quicpingli@codeaurora.org>