rusticl/device: fix default device enumeration

There were a couple of issues this fixes:
 - Using the env "RUSTICL_DEVICE_TYPE" variable made no device be default.
 - If multiple GPUs were detected, multiple devices were default.
 - If zink was used, no default device was advertized either.

The spec requires that there must be one and only one default device.

Cc: mesa-stable
Reviewed-by @LingMan

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32875>
3 files changed