drm/msm/dpu: Cleanup dpu plane interface

Remove unused functions from dpu plane interface
and unused variables from dpu plane state structure.

Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>