disp: msm: dsi: correct external dsi bridge interface number

Get the interface number from dsi panel timing node, if no
timing node was set, set the number as ext_bridge_num.

Change-Id: I1f36df6b5780f6eb1ef42e5b260cbdb16ec61c15
Signed-off-by: Boyuan Yan <boyuyan@qti.qualcomm.com>