drm/msm/sde: add secure fb translation mode for writeback

Add secure fb translation mode property to writeback connector
to support secure writeback. Writeback buffer can be non-secure
and secure.

Change-Id: Icdf915a139eb508db76a58cd2f1f58b2943bc541
Signed-off-by: Alan Kwong <akwong@codeaurora.org>