drm/amd/display: Fix FBC topology change

With FBC enabled there was a potential null-deref
on topology change due to hardcorded pipe index.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>