drm/msm: Don't subclass drm_atomic_state anymore

With the addition of "private_objs" in drm_atomic_state, we no longer
need to subclass drm_atomic_state to store state of share resources
that don't perfectly fit within planes/crtc/connector state information.
We can now save this state within drm_atomic_state itself using
the private objects.

Remove the infrastructure that allowed subclassing of drm_atomic_state
in the driver.

Changes in v3:
- Added to the msm atomic helper patch set
Changes in v4:
- None

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