msm: board: reserve mdp writeback memory

The memory allocation happens on the display mdp driver side. This
separation of writeback buffers from the frame buffer is mainly for
content security.

Change-Id: I404b5f8db01dd5ed940916d7d46a8ace2d295e3a
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
8 files changed