drm/msm/dsi: Use the new setup_encoder function in attach_dsi_device

Now that we have a function to call set_encoder_mode() for us, use it.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617201301.133275-8-sean@poorly.run