wayland/buffer: Don't assert mutter supports all shm formats

On big endian architectures the mapping of drm formats to cogl formats is significantly trimmed compared to on little endian architectures.

meta_wayland_init_shm tries a bunch of formats, including some that just aren't mapped on e.g. s390x.

The code asserts the mapping will exist, however, leading to crashes when Xwayland starts.

This commit makes failure to find a mapping non-fatal.

Suggested by Jonas Ådahl.