drm/msm/dpu: Remove unused enum and comment from dpu mdss

Remove enum dpu_iommu_domain from dpu mdss as its unused.

Remove unnecessary comment for variable which is already
removed.

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