drm/msm/mdp5: Remove old kms init/destroy funcs

With the new kms_init/destroy funcs in place for MDP5, we can get rid of
the old kms funcs. Some members of the mdp5_kms struct also become
redundant, so we remove those too.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>