disp: msm: sde: fix dsi bridge init

Initialization of DSI external bridge must be done within
the initialization of DSI displays. The current method
has the drm_ext_bridge_init being called outside the
loop of DSI displays, and it will fail as the display,
encoder and connector handle are not present.
This fixes the regresion caused by
commit edef6ae0 ("disp: msm: dsi: snapshot of dsi from
4.14 to 4.19").

Change-Id: I50d0a303c2c8f4323e46cf14df1b071ebae80ceb
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>