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.

Source/Core/VideoBackends/D3D/D3DState.h

@@ -234,6 +234,9 @@ public:
 		}
 	}
 
+	// removes currently set texture from all slots, returns mask of previously bound slots
+	u32 StateManager::UnsetTexture(ID3D11ShaderResourceView* srv);
+
 	// call this immediately before any drawing operation or to explicitly apply pending resource state changes
 	void Apply();