libavutil/hwcontext_qsv: add usage child_device_type argument to explicitly select d3d11va/DX11 device type

UPD: Rebase of last patch set over current master and use DX9 as default device type.

Makes selection of dxva2/DX9 device type by default as before with explicit d3d11va/DX11 usage to cover more HW configurations.
Added warning message to expect changing default device type in the future.

Fixes TGL / AV1 decode as requires DX11 with explicit DX11 type
selection.

Add headless/multi adapter support and fixes:
    https://trac.ffmpeg.org/ticket/7511
    https://trac.ffmpeg.org/ticket/6827
    http://ffmpeg.org/pipermail/ffmpeg-trac/2017-November/041901.html
    https://trac.ffmpeg.org/ticket/7933
    https://github.com/InitialForce/FFmpeg/commit/338fbcd5bba1de0e1b3e3bad8985eee2fdfbeca1
    https://github.com/jellyfin/jellyfin/issues/2626#issuecomment-602153952

Any other fixes are welcome including OpenCL interop patch since I don't have proper setup to validate this use case

Decoding, encoding, transcoding have been validated.

child_device_type option is responsible for d3d11va/dxva2 device selection

Usage examples:

DirectX 11:
    -init_hw_device qsv:hw,child_device_type=d3d11va
    -init_hw_device qsv:hw,child_device_type=d3d11va,child_device=0
OR
    -init_hw_device d3d11va=dx -init_hw_device qsv@dx

DirectX 9 is still supported but requires explicit selection:
    -init_hw_device qsv:hw,child_device_type=dxva2
OR
    -init_hw_device dxva2=dx -init_hw_device qsv@dx
Signed-off-by: Artem Galin <artem.galin@intel.com>