drm/amd/display: add null checks before logging

Adding NULL checks to various parameters in log_tf, to avoid
nullptr errors

Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Nikola Cornij <Nikola.Cornij@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>