dxva: preparations for new hwaccel API

The actual hwaccel code will need to access an internal context instead
of avctx->hwaccel_context, so add a new DXVA_CONTEXT() macro, that will
dispatch between the "old" external and the new internal context.

Also, the new API requires a new D3D11 pixfmt, so all places which check
for the pixfmt need to be adjusted. Introduce a ff_dxva2_is_d3d11()
function, which does the check.

Merges Libav commit 4dec101acc393fbfe9a8ce0237b9efbae3f20139.
Adds changes to vp9 over the Libav patch.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
