drm/msm/shd: add shared display support

Add support to shared display for panel stacking and split screen.
The current DRM framework only allows one atomic commit for
each display, this change will create virtual display pipelines
based on the physical display pipeline, and each virtual display
can support independent async atomic commit.

CRs-Fixed: 2391344
Change-Id: I06c55b6f894eef6ded852fc315c84af8568d8a75
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>