drm/amd/display: Create dm_atomic_state

We really want to use the new private_atomic_state but can't right now
as we have to maintain some backward compatibility to older kernels. For
now let's follow Intel's approach and extend the drm_atomic_state.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>