drm/amd/display: add null check for cursor functions

Signed-off-by: Yue Hin Lau <Yuehin.Lau@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>