hw/display: Restrict virtio-gpu-udmabuf stubs to !Linux

hw/display: Restrict virtio-gpu-udmabuf stubs to !Linux

When using qemu configured with --enabled-modules, the
generic stubs are used instead of the module symbols:

  qemu-system-x86_64: -device virtio-vga,blob=on: cannot enable blob resources without udmabuf

Restrict the stubs to Linux and only link them when
CONFIG_VIRTIO_GPU is disabled (only the modularized
version is available when it is enabled).

Reported-by: Maxim R. <mrom06@ya.ru>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/553
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210823100454.615816-2-philmd@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>