drm/msm/mdp5: drop split display support

drm/msm/mdp5: drop split display support

The MSM DSI driver has dropped support for calling
mdp_kms_funcs::set_split_display() callback. Drop corresponding callback
from the mdp5 driver together with the rest of the infrastructure.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/561698/
Link: https://lore.kernel.org/r/20231009205727.2781802-6-dmitry.baryshkov@linaro.org