[3.0] direct3d11: fix uninitialized src_fmt when render with external device

In the direct3d11 module, CommonInit won't execute if external_device is true, thus leaving vout_display_sys_win32_t.src_fmt = NULL. The module crashes soon after when it tries to use src_fmt in UpdateRects. The solution is to initialize src_fmt in direct3d11 when using external device.