msdk: allocator: libva: check if it's already using dmabuf when mapping

As long as we negotiate the "DMABuf" capsfeatures for now, map can't be
working. So we need to confirm not to do it if using DMABuf memory.

https://bugzilla.gnome.org/show_bug.cgi?id=793707

sys/msdk/gstmsdkallocator_libva.c

@@ -239,6 +239,11 @@ gst_msdk_frame_lock (mfxHDL pthis, mfxMemId mid, mfxFrameData * data)
   va_surface = mem_id->surface;
   dpy = gst_msdk_context_get_handle (context);
 
+  if (mem_id->info.mem_type == VA_SURFACE_ATTRIB_MEM_TYPE_DRM_PRIME) {
+    GST_WARNING ("Couldn't map the buffer since dmabuf is already in use");
+    return MFX_ERR_LOCK_MEMORY;
+  }
+
   if (mem_id->fourcc != MFX_FOURCC_P8) {
     va_status = vaDeriveImage (dpy, *va_surface, &mem_id->image);
     status = gst_msdk_get_mfx_status_from_va_status (va_status);