drm/sde: assign different default zpos for each plane

zpos is a customized plane property. Not all of DRM client will set
it. Driver needs to assign correct default zpos value for each plane
in order to support blend stage for hardware.

Change-Id: I3f463ba5556c50488bf8bec9c40c578c2e65bd5d
Signed-off-by: Jin Li <jinl@codeaurora.org>