drm/msm/dsi-staging: move ext bridge init from postopen to hw_init

Move ext bridge init from postopen to hw_init to avoid file open racing
issue, also make sure ext bridge is functional during cont splash.

Add ext bridge checking in bind stage, defer if ext bridge is not attached.

CRs-Fixed: 2393741
Change-Id: Idfab267a0f8741e45bcaf41e92e1bf28b321f627
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>