drm/amd/display: Added missing null checks

drm/amd/display: Added missing null checks

[why&how]
Add the missing null check before dereference for dc_stream_status*

Fixes: 2d5bb791e24f ("drm/amd/display: Implement update_planes_and_stream_v3 sequence")
Reviewed-by: Josip Pavic <josip.pavic@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Sohaib Nadeem <sohaib.nadeem@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>