drm/sched: Barriers are needed for entity->last_scheduled

It might be good enough on x86 with just READ_ONCE, but the write side
should then at least be WRITE_ONCE because x86 has total store order.

It's definitely not enough on arm.

Fix this proplery, which means
- explain the need for the barrier in both places
- point at the other side in each comment

Also pull out the !sched_list case as the first check, so that the
code flow is clearer.

While at it sprinkle some comments around because it was very
non-obvious to me what's actually going on here and why.

Note that we really need full barriers here, at first I thought
store-release and load-acquire on ->last_scheduled would be enough,
but we actually requiring ordering between that and the queue state.

v2: Put smp_rmp() in the right place and fix up comment (Andrey)

Reviewed-by: Christian König <[email protected]>
Acked-by: Melissa Wen <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: "Christian König" <[email protected]>
Cc: Steven Price <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Andrey Grodzovsky <[email protected]>
Cc: Lee Jones <[email protected]>
Cc: Boris Brezillon <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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