drm/msm/sde: add hardware driver support for writeback interface

Writeback block is used to output composited images to memory.
This patch adds support to the hardware driver layer such that
writeback hardware block can be controlled using hardware
abstraction api common across families of hardware.

Change-Id: I92a69f1d564ee87e3f040cbef5a0302468599440
Signed-off-by: Alan Kwong <akwong@codeaurora.org>