drm/msm/dpu: Remove an unused enum

enum dpu_ad isn't used and can be safely removed.

Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>

Signed-off-by: Rob Clark <robdclark@gmail.com>