util: Change os_same_file_description return type from bool to int

util: Change os_same_file_description return type from bool to int

This allows communicating that it wasn't possible to determine whether
the two file descriptors reference the same file description. When
that's the case, log a warning in the amdgpu winsys.

In turn, remove the corresponding debugging output from the fallback
os_same_file_description implementation. It depends on the caller if
false negatives are problematic or not.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3879>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3879>