drm/msm: ensure display driver probe completion

This change is done so that in non display platform
indefinite wait is not introduced, however for platforms where
display driver is probed asynchronously we still
wait for completion.

Change-Id: I14932ee2c0e52330975529ca0be34b6e2521d05e
Signed-off-by: Rahul Sharma <rahsha@codeaurora.org>