drm/sched: Implement RR via FIFO

Round-robin being the non-default policy and unclear how much it is used,
we can notice that it can be implemented using the FIFO data structures if
we only invent a fake submit timestamp which is monotonically increasing
inside drm_sched_rq instances.

So instead of remembering which was the last entity the scheduler worker
picked we can simply bump the picked one to the bottom of the tree, which
ensures round-robin behaviour between all active queued jobs.

If the picked job was the last from a given entity, we remember the
assigned fake timestamp and use it to re-insert the job once it re-joins
the queue. This ensures the job neither overtakes all already queued jobs,
neither it goes last. Instead it keeps the position after the currently
queued jobs and before the ones which haven't yet been queued at the point
the entity left the queue.

Advantage is that we can consolidate to a single code path and remove a
bunch of code. Downside is round-robin mode now needs to lock on the job
pop path but that should not have a measurable performance impact.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Philipp Stanner <phasta@kernel.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Tested-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20260417103744.76020-7-tvrtko.ursulin@igalia.com

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