drm/i915/execlists: Mark up data-races in virtual engines

The virtual engine passes tokens back and forth to its backing physical
engines.

[   57.372993] BUG: KCSAN: data-race in execlists_dequeue [i915] / virtual_submission_tasklet [i915]
[   57.373012]
[   57.373023] write to 0xffff8881f47324c0 of 4 bytes by interrupt on cpu 2:
[   57.373241]  execlists_dequeue+0x6fa/0x2150 [i915]
[   57.373458]  __execlists_submission_tasklet+0x48/0x60 [i915]
[   57.373677]  execlists_submission_tasklet+0xd3/0x170 [i915]
[   57.373694]  tasklet_action_common.isra.0+0x42/0xa0
[   57.373709]  __do_softirq+0xd7/0x2cd
[   57.373723]  irq_exit+0xbe/0xe0
[   57.373735]  do_IRQ+0x51/0x100
[   57.373748]  ret_from_intr+0x0/0x1c
[   57.373963]  engine_retire+0x89/0xe0 [i915]
[   57.373977]  process_one_work+0x3b1/0x690
[   57.373990]  worker_thread+0x80/0x670
[   57.374004]  kthread+0x19a/0x1e0
[   57.374017]  ret_from_fork+0x1f/0x30
[   57.374027]
[   57.374038] read to 0xffff8881f47324c0 of 4 bytes by interrupt on cpu 3:
[   57.374256]  virtual_submission_tasklet+0x27/0x5a0 [i915]
[   57.374273]  tasklet_action_common.isra.0+0x42/0xa0
[   57.374288]  __do_softirq+0xd7/0x2cd
[   57.374302]  run_ksoftirqd+0x15/0x20
[   57.374315]  smpboot_thread_fn+0x1ab/0x300
[   57.374329]  kthread+0x19a/0x1e0
[   57.374342]  ret_from_fork+0x1f/0x30

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200310141320.24149-3-chris@chris-wilson.co.uk