D3D: Fixed D3D validation error during EFB to texture copy

Texture was being bound as a render target while still being set as a shader resource.
D3D automatically unbinds the SRV in this case and generates a validation error.
The fix is to manually unbind SRV, render into it and then re-bind to old slots.

This was responsible for https://code.google.com/p/dolphin-emu/issues/detail?id=7929 and quite likely https://forums.dolphin-emu.org/Thread-4-0-4583-d3d-regression?pid=348967