drm/i915/gt: Mark up racy reads for intel_context.inflight

When being used across multiple real engines inside a virtual engine,
the intel_context.inflight is updated atomically, and so we must
annotate the racy read from outside the owning context.

[11142.482846] BUG: KCSAN: data-race in __execlists_submission_tasklet [i915] / __execlists_submission_tasklet [i915]
[11142.482867]
[11142.482878] write (marked) to 0xffff8881f257b5e0 of 8 bytes by interrupt on cpu 2:
[11142.483107]  __execlists_submission_tasklet+0x1d33/0x2120 [i915]
[11142.483336]  execlists_submission_tasklet+0xd3/0x170 [i915]
[11142.483355]  tasklet_action_common.isra.0+0x42/0xa0
[11142.483371]  __do_softirq+0xd7/0x2cd
[11142.483384]  irq_exit+0xbe/0xe0
[11142.483401]  do_IRQ+0x51/0x100
[11142.483424]  ret_from_intr+0x0/0x1c
[11142.483446]  do_idle+0x133/0x1f0
[11142.483465]  cpu_startup_entry+0x14/0x16
[11142.483483]  start_secondary+0x120/0x180
[11142.483498]  secondary_startup_64+0xa4/0xb0
[11142.483512]
[11142.483528] read to 0xffff8881f257b5e0 of 8 bytes by interrupt on cpu 1:
[11142.483755]  __execlists_submission_tasklet+0x14e/0x2120 [i915]
[11142.483981]  execlists_submission_tasklet+0xd3/0x170 [i915]
[11142.483999]  tasklet_action_common.isra.0+0x42/0xa0
[11142.484014]  __do_softirq+0xd7/0x2cd
[11142.484028]  do_softirq_own_stack+0x2a/0x40
[11142.484046]  do_softirq.part.0+0x26/0x30
[11142.484071]  __local_bh_enable_ip+0x46/0x50
[11142.484299]  i915_gem_do_execbuffer+0x39c1/0x4e50 [i915]
[11142.484528]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
[11142.484546]  drm_ioctl_kernel+0xe4/0x120
[11142.484559]  drm_ioctl+0x297/0x4c7
[11142.484572]  ksys_ioctl+0x89/0xb0
[11142.484586]  __x64_sys_ioctl+0x42/0x60
[11142.484610]  do_syscall_64+0x6e/0x2c0
[11142.484627]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

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/20200310141320.24149-1-chris@chris-wilson.co.uk