ffmpeg: dxva2_h264: set the correct ref frame index in the long slice struct

The latest H.264 DXVA specification states that the index in this
structure should refer to a valid entry in the RefFrameList of the picture
parameter structure, and not to the actual surface index.

Fixes H.264 DXVA2 decoding on recent Intel GPUs (tested on Sandy and Ivy)
ffmpeg: dxva2_h264: add a workaround for old intel GPUs

Old Intel GPUs expect the reference frame index to the actual surface,
instead of the index into RefFrameList as specified by the spec.

This workaround should be set when using one of the "ClearVideo" decoder
devices.
[dxva] Add support workaround for Intel H.264 VLD, no FGT, ClearVideo decoder.
[dxva] Prefer standard Microsoft H.264 device instead of Intel H.264 VLD, no FGT (ClearVideo).

This fix h264 decoding with refs > 11 on recent Intel GPUs (SNB/IVB). The latest Intel drivers for IVB/Haswell already has no support old Intel H.264 VLD, no FGT (ClearVideo) device, so this actually needs for SNB and older.
Merge pull request #4228 from afedchin/dxva_h264_intel

[DXVA] Fix h264 deconding with refs > 11 on recent Intel GPUs (SNB/IVB)