mesa/glthread: fallback to sync if count validation fails

mesa/glthread: fallback to sync if count validation fails

The old code would sync and then throw a cryptic error message.
There is no need for a custom error, we can just fallback to
the real function and have it do proper validation.

Fixes piglit test:
glsl-uniform-out-of-bounds

Which was returning the wrong error code.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>