drm/amd/display: Add null pointer filter

[Why & How]
Add null pointer filter for logical integrity.

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Signed-off-by: Jing Zhou <Jing.Zhou@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>