drm/amd/display: Use new DRM API where possible

To conform to DRM's new API, we should not be accessing a DRM object's
internal state directly. Rather, the DRM for_each_old/new_* iterators,
and drm_atomic_get_old/new_* interface should be used.

This is an ongoing process. For now, update the DRM-facing atomic
functions, where the atomic state object is given.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>