backend/session: allow wlr_session_find_gpus to return an error

Sometimes wlr_session_find_gpus will encounter an error. This is
different from finding zero GPUs.

On error, wlr_session_find_gpus already returns -1. However, this is
casted to size_t, so callers uncorrectly assume this is a success.

Instead, make wlr_session_find_gpus return a ssize_t and allow callers
to handle the error accordingly.

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