D3D: Couple of small optimizations

* Vertex and index data in one buffer
* Pixel shader resources are not reset unnecessarily

Vertex and index buffer are usually both updated for every draw call.
We can avoid some driver overhead related to mapping/unmapping by using just one buffer.
This is the same optimization as Galop1n did in https://github.com/dolphin-emu/dolphin/pull/438.

Yup, but I'd like to implement a more comprehensive API cache/filtering, which would expand the scope of this PR too much.