drm/msm/mdp5: atomic

Convert mdp5 over to atomic helpers.  Extend/wrap drm_plane_state to
track plane zpos and to keep track of the needed when applying the
atomic update.  In mdp5's plane->atomic_check() we also need to check
for updates which require SMP reallocation, in order to trigger full
modeset.

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