drm/i915/gt: Carefully order virtual_submission_tasklet

During the virtual engine's submission tasklet, we take the request and
insert into the submission queue on each of our siblings. This seems
quite simply, and so no problems with ordering. However, the sibling
execlists' submission tasklets may run concurrently with the virtual
engine's tasklet, submitting the request to HW before the virtual
finishes its task of telling all the siblings. If this happens, the
sibling tasklet may *reorder* the ve->sibling[] array that the virtual
engine tasklet is processing. This can *only* reorder within the
elements already processed by the virtual engine, nevertheless the
race is detected by KCSAN:

[  185.580014] BUG: KCSAN: data-race in execlists_dequeue [i915] / virtual_submission_tasklet [i915]
[  185.580054]
[  185.580076] write to 0xffff8881f1919860 of 8 bytes by interrupt on cpu 2:
[  185.580553]  execlists_dequeue+0x6ad/0x1600 [i915]
[  185.581044]  __execlists_submission_tasklet+0x48/0x60 [i915]
[  185.581517]  execlists_submission_tasklet+0xd3/0x170 [i915]
[  185.581554]  tasklet_action_common.isra.0+0x42/0x90
[  185.581585]  __do_softirq+0xc8/0x206
[  185.581613]  run_ksoftirqd+0x15/0x20
[  185.581641]  smpboot_thread_fn+0x15a/0x270
[  185.581669]  kthread+0x19a/0x1e0
[  185.581695]  ret_from_fork+0x1f/0x30
[  185.581717]
[  185.581736] read to 0xffff8881f1919860 of 8 bytes by interrupt on cpu 0:
[  185.582231]  virtual_submission_tasklet+0x10e/0x5c0 [i915]
[  185.582265]  tasklet_action_common.isra.0+0x42/0x90
[  185.582291]  __do_softirq+0xc8/0x206
[  185.582315]  run_ksoftirqd+0x15/0x20
[  185.582340]  smpboot_thread_fn+0x15a/0x270
[  185.582368]  kthread+0x19a/0x1e0
[  185.582395]  ret_from_fork+0x1f/0x30
[  185.582417]

We can prevent this race by checking for the ve->request after looking
up the sibling array.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200423115315.26825-1-chris@chris-wilson.co.uk