Revert "wayland-drm: constify the callbacks struct"

Revert "wayland-drm: constify the callbacks struct"

The wayland-drm callback struct is referenced, rather than duplicated,
inside wayland-drm. Constifying this struct involved moving it on to the
stack; as a result, starting any EGL client on Wayland called into
random stack memory, and killed the compositor.

This reverts commit 1d0be5b3fe548ee33d4520092f583c76d42510a6 and
39d539e321c6c97433a15660c9d9a20ad8657ff0.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Krzysztof Sobiecki <sobkas@gmail.com>
Fixes: 1d0be5b3fe54 ("wayland-drm: constify the callbacks struct")