drm/amd/display: check for null before calling is_blanked

Signed-off-by: Yue Hin Lau <Yuehin.Lau@amd.com>
Reviewed-by: Eric Bernstein <Eric.Bernstein@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>