drm/msm/dsi-staging: do not init ext bridge for dsi panel

external bridge is not needed in case of dsi panel,
check if there is ext_bridge_mode defined.

Signed-off-by: Chong Gu <chongu@codeaurora.org>
Change-Id: I8db5d0cb5ae4521c2e7174a39522fb5d0340d9de