drm/msm/dsi: Move setup_encoder to modeset_init

Now that the panel probe/setup is in the modeset path, we can call
dsi_manager_setup_encoder() in a common place for both internal and
external bridge setups.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617201301.133275-10-sean@poorly.run