GSdx-D3D11: Avoid undefined behavior in swapchain

PCSX2 sends a negative value (-1) to GSdx when adaptive mode is
specified for Vsync, this mode is exclusive to OpenGL at the moment
and is unimplemented on the D3D11 renderer. Also the present function
of swapchain only accepts values from 0 to 4 as parameter, hence
passing negative values to the function is undefined behavior.

So let's fallback to standard synchronization method on D3D11 when
PCSX2 requests for adaptive mode.

plugins/GSdx/GSDevice11.cpp

@@ -432,6 +432,11 @@ void GSDevice11::SetExclusive(bool isExcl)
 	}
 }
 
+void GSDevice11::SetVSync(int vsync)
+{
+	m_vsync = vsync ? 1 : 0;
+}
+
 void GSDevice11::Flip()
 {
 	m_swapchain->Present(m_vsync, 0);

plugins/GSdx/GSDevice11.h

@@ -170,6 +170,7 @@ public:
 	bool Create(const std::shared_ptr<GSWnd> &wnd);
 	bool Reset(int w, int h);
 	void Flip();
+	void SetVSync(int vsync) final;
 
 	void SetExclusive(bool isExcl);