Commit 22723ca37 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.
(cherry picked from commit 54709c16b5)
b2a035348b