msm: mdss: abstract the writeback block

There are different programming interfaces in different hardware
revisions. This change abstracts the writeback block, so that
the client does not need to deal with the differences.

Change-Id: I3f49ee1f2e003655dbef0d3281d9c9ad95cdb327
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>