drm/msm/dpu: Remove unused functions

Remove some unused container_of() helper functions.

v3: No changes
v2: Retained still used helper functions in the name of readability

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