drm/msm/dsi-staging: Add external DSI bridge support

Add external DSI to DP/HDMI bridge support to allow upstream driver
in drm/bridge work with dsi-staging driver without any change.

CRs-Fixed: 2242018
Change-Id: I92302314da7a1c964a46b52c3315a9d436814dc8
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>