[VAAPI AV1 Encode] Encoder doesn't check VAConfigAttribEncMaxRefFrames

On official ffmpeg (with the upstreamed AV1 encode patch), the encoder seems to be generating bitstreams using always 2 reference frames set to LAST/GOLDEN for P, even when VAConfigAttribEncMaxRefFrames.L0 reports a lower limit (e.g max 1 L0 reference frame).

@feiwan1 Is this something that could be checked so only the LAST reference is used for P when VAConfigAttribEncMaxRefFrames.L0 = 1 ? This affects the vaon12 VA driver which has encode AV1 support over different hardware abstracted through D3D12 reporting different support for VAConfigAttribEncMaxRefFrames.

Thanks!