drm/msm/dsi-staging: Do not bind ext bridge when dsi panel was set

HDK device support both ext bridge and DSI panel,
we need not bind ext bridge when DSI panel was set.

Change-Id: I3bcdce6e4110c5ebe063cbcf767368d43b8b11ba
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>