drm/msm/sde: add panel stacking support

To support transparent panel stacking, application will pass non-panel-
stacking buffer roi to kernel and let kernel to extend to panel-stacking
size.  A special flag is defined on source config for this purpose.

Change-Id: I59eb290f0f7b92744972391d7db743e2da12300b
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Rahul Sharma <rahsha@codeaurora.org>