drm/i915/execlists: Mark up read of i915_request.fence.flags

[  145.927961] BUG: KCSAN: data-race in can_merge_rq [i915] / signal_irq_work [i915]
[  145.927980]
[  145.927992] write (marked) to 0xffff8881e513fab0 of 8 bytes by interrupt on cpu 2:
[  145.928250]  signal_irq_work+0x134/0x640 [i915]
[  145.928268]  irq_work_run_list+0xd7/0x120
[  145.928283]  irq_work_run+0x1d/0x50
[  145.928300]  smp_irq_work_interrupt+0x21/0x30
[  145.928328]  irq_work_interrupt+0xf/0x20
[  145.928356]  _raw_spin_unlock_irqrestore+0x34/0x40
[  145.928596]  execlists_submission_tasklet+0xde/0x170 [i915]
[  145.928616]  tasklet_action_common.isra.0+0x42/0xa0
[  145.928632]  __do_softirq+0xd7/0x2cd
[  145.928646]  irq_exit+0xbe/0xe0
[  145.928665]  do_IRQ+0x51/0x100
[  145.928684]  ret_from_intr+0x0/0x1c
[  145.928699]  schedule+0x0/0xb0
[  145.928719]  worker_thread+0x194/0x670
[  145.928743]  kthread+0x19a/0x1e0
[  145.928765]  ret_from_fork+0x1f/0x30
[  145.928784]
[  145.928796] read to 0xffff8881e513fab0 of 8 bytes by task 738 on cpu 1:
[  145.929046]  can_merge_rq+0xb1/0x100 [i915]
[  145.929282]  __execlists_submission_tasklet+0x866/0x25a0 [i915]
[  145.929518]  execlists_submit_request+0x2a4/0x2b0 [i915]
[  145.929758]  submit_notify+0x8f/0xc0 [i915]
[  145.929989]  __i915_sw_fence_complete+0x5d/0x3e0 [i915]
[  145.930221]  i915_sw_fence_complete+0x58/0x80 [i915]
[  145.930453]  i915_sw_fence_commit+0x16/0x20 [i915]
[  145.930698]  __i915_request_queue+0x60/0x70 [i915]
[  145.930935]  i915_gem_do_execbuffer+0x3997/0x4c20 [i915]
[  145.931175]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
[  145.931194]  drm_ioctl_kernel+0xe4/0x120
[  145.931208]  drm_ioctl+0x297/0x4c7
[  145.931222]  ksys_ioctl+0x89/0xb0
[  145.931238]  __x64_sys_ioctl+0x42/0x60
[  145.931260]  do_syscall_64+0x6e/0x2c0
[  145.931275]  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/20200309110934.868-4-chris@chris-wilson.co.uk