drm/msm/sde: stage layer with zorder 0 as base layer

Add support to stage layer with zorder 0 as base
layer and stage borderfill only during null commit.

Change-Id: I54356c1b7834227cc3da00c211e71ac5816ce51a
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>
Signed-off-by: Prashant Singh <prasin@codeaurora.org>