wayland: Realize dmabuf buffers before trying to attach them

Commit 22723ca3 moved buffer realization to meta_wayland_surface_commit() so that it wouldn't be part of meta_wayland_buffer_attach().

However, creation of dmabuf buffers would call into meta_wayland_buffer_attach() directly without realizing the buffer first. attach() would then fail and mutter would effectively shut down any clients using the zwp_linux_dmabuf protocol (note that if such client was Xwayland, mutter itself would shut down as well).

Add the missing bit in order to make zwp_linux_dmabuf protocol work again.