drm/amd/display: Unify DRM state variable namings.

Use new_*_state and old_*_state for their respective new/old DRM object
states.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>