drm/msm/sde: add shared display support

Add support to shared display for panel stacking and split screen
application.

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.

With the change user mode application can setup and update
display on each virtual display and share the same physical port.
No mode switch is allowed on virtual display. When all virtual
displays are powered off,  the physical display will also power off.

Change-Id: Ibf9a7dd1a96c59b6db9891d826cb4f11b409696c
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Rahul Sharma <rahsha@codeaurora.org>