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.

This two are likely very often redundant. Do you want to cache them for an additional speedup? (Or was I just not able to find this code?)