wayland/dma-buf: Enable modifiers by default on non-native backend

If the used EGL backend supports it. In practice this should currently
only affect the nested backend.

Enabling modifiers can help with app development. An example is
`weston-simple-dmabuf-v4l`, which requires the linear modifier to be
available.

Note that Weston behaves similar already.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2972>