d3d11videosink: Fix error on pause and play

The show_frame() can be called without prepare() call on paused to
playing state change. Thus the prepared buffer should not be cleared
on show_frame()

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4700>

subprojects/gst-plugins-bad/sys/d3d11/gstd3d11videosink.cpp

@@ -1452,8 +1452,6 @@ gst_d3d11_video_sink_show_frame (GstVideoSink * sink, GstBuffer * buf)
     ret = GST_FLOW_ERROR;
   }
 
-  gst_clear_buffer (&self->prepared_buffer);
-
   return ret;
 }