drm/msm/dpu: Clean up plane atomic disable/update

Removes unnecessary checks from dpu_plane_atomic_disable, old_state
argument for both dpu_plane_atomic_disable and
dpu_plane_sspp_atomic_update is removed as it is no longer used.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Bruce Wang <bzwang@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>