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

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

Now that the TE setup has been moved to prepare_for_kickoff(),  we have
not prepare_commit() callbacks left. This makes dpu_encoder_prepare_commit()
do nothing. Remove prepare_commit() from DPU driver.

Changes in V3:
- Reworded commit message to be more clear
- Corrected spelling mistake in commit message

Changes in V4:
- Reworded commit message for clarity

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/523606/
Link: https://lore.kernel.org/r/20230221184256.1436-4-quic_jesszhan@quicinc.com
[DB: fixed merge conflict]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>