drm/msm/mdp4: Remove empty prepare_commit() function

Remove empty prepare_commit() function from MDP4 driver.

Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/523608/
Link: https://lore.kernel.org/r/20230221184256.1436-5-quic_jesszhan@quicinc.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>