mesa: remove fallback RefCount == 0 pattern

mesa: remove fallback RefCount == 0 pattern

We should never get here if this is 0 unless there is a
bug. Replace the check with an assert.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>