drm/msm/dsi-staging: add split DSI support with external bridge

Add split DSI support with external bridge. Multiple endpoints
are supported to attach all the mipi_dsi bridges.

Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Change-Id: Ibf231f06d7fa493b5d0612bb4a69cb04f3579b0d