drm/msm: dpu: Make dpu_plane_sspp_atomic_update() void

All of the checks in dpu_plane_sspp_atomic_update() are impossible, so
remove them and make the function void. This removes the need to error
check in dpu_plane_atomic_update(). Additionally, remove impossible checks
in dpu_plane_atomic_update().

Changes in v2:
- None

Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>