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

Use dm_new_*_state and dm_old_*_state for their respective amdgpu_dm new
and old object states. Helps with readability, and enforces use of new
DRM api (choose either new, or old).
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>