drm/msm/dpu: Remove unused call in wait_for_commit_done

The call to dpu_encoder_phys_cmd_prepare_for_kickoff is useless as
it's unused because the serialize_wait4pp variable is never set to
true by .. anything, literally: remove the call.
While at it, also reduce indentation by inverting the check for
dpu_encoder_phys_cmd_is_master.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>