drm/msm/mdp5: add skeletal mdp5_state

Add basic state duplication/apply mechanism.  Following commits will
move actual global hw state into this.

The state_lock allows multiple concurrent updates to proceed as long as
they don't both try to alter global state.  The ww_mutex mechanism will
trigger backoff in case of deadlock between multiple threads trying to
update state.

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