wayland/dma-buf: Always send modifiers when using surfaceless

We'll never scan out, which is why ADDFB2 is required otherwise, and we
won't enable the DMA buffer extension if
'EGL_EXT_image_dma_buf_import_modifiers' is missing, so send modifiers
in this case.

This also happens to avoid crashing when the GPU is null, since we'd
otherwise attempt to dereference it.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2578>