radeonsi: enable glthread by default

Let's enable it and see what happens. This should mostly be a win.

Reviewed-by: Mihai Preda <mhpreda@gmail.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <!18026>
191 jobs for !18026 with si-glthread-by-default in 118 minutes and 46 seconds (queued for 8 seconds)
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