d3d12: AV1 Encode fixes for latest apps
Three small fixes for AV1 Encode that show up when updating testing to the latest cartwheel-ffmpeg available.
d3d12: Fix usage of D3D12_VIDEO_ENCODER_RATE_CONTROL_FLAG, was using D3D12_VIDEO_ENCODER_SUPPORT_FLAG wrongly instead
d3d12: Only set reduced_tx_set when supported by D3D12 caps (no libva caps for reduced_tx_set to map to)
d3d12: Correct tx_mode_support reporting as specified in libva spec and now rectified in apps