hwcontext_d3d11va: implement av_hwdevice_get_hwframe_constraints()

hwcontext_d3d11va: implement av_hwdevice_get_hwframe_constraints()

D3D11 has rather fine grained per format capabilities for different uses
that can be queried at runtime. Since we don't know what the user wants
to do with the formats when av_hwdevice_get_hwframe_constraints() is
called, we simply return all formats that have the most basic support.