drm/amd/display: Only finalize atomic_obj if it was initialized

[Why]
If amdgpu_dm failed to initalize before amdgpu_dm_initialize_drm_device()
completed then freeing atomic_obj will lead to list corruption.

[How]
Check if atomic_obj state is initialized before trying to free.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>