drm/msm/dsi: stop calling set_split_display

Since the commit 8b03ad30 ("drm/msm/dsi: Use one connector for dual
DSI mode"), the second DSI host in the bonded pair will not be
associated with the encoder and will not get the bridges, thus making
condition in msm_dsi_manager_set_split_display() always false.

Technically that change broke bonded DSI support in the MDP5 driver. But
since nobody complained in the last 5.5 years, it seems that nobody
cares enough.

Drop the msm_dsi_manager_set_split_display() completely and stop calling
the set_split_display() KMS callback. Also remove the
msm_dsi::external_bridge field which was only used by the mentioned
function.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/561695/
Link: https://lore.kernel.org/r/20231009205727.2781802-4-dmitry.baryshkov@linaro.org