drm/msm/shd: fix plane stage check in shared display

Fix plane stage setting in shared display and change debug level for stage
validation. Shared display has less stages than normal display and might
fail stage validation. Change debug level for stage validation to avoid
error messages flooding the console.

Change-Id: I226aafcf124af2212bdafcb02f87c99538b956cf
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>