ui/gtk: graphic_hw_gl_flushed after closing dmabuf->fence_fd

The dmabuf often becomes invalid right after unblocking pipeline
and graphic_hw_gl_flushed in case a new scanout blob is submitted
because the dmabuf associated with the current guest scanout is
freed after swapping.

So both graphic_hw_gl_block and graphic_hw_gl_flushed should be
executed after closing fence_fd for the current dmabuf.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Message-Id: <20211121172237.14937-1-dongwon.kim@intel.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>