drm/i915/execlsts: Mark up racy inspection of current i915_request priority

[  120.176548] BUG: KCSAN: data-race in __i915_schedule [i915] / effective_prio [i915]
[  120.176566]
[  120.176577] write to 0xffff8881e35e6540 of 4 bytes by task 730 on cpu 3:
[  120.176792]  __i915_schedule+0x63e/0x920 [i915]
[  120.177007]  __bump_priority+0x63/0x80 [i915]
[  120.177220]  __i915_sched_node_add_dependency+0x258/0x300 [i915]
[  120.177438]  i915_sched_node_add_dependency+0x50/0xa0 [i915]
[  120.177654]  i915_request_await_dma_fence+0x1da/0x530 [i915]
[  120.177867]  i915_request_await_object+0x2fe/0x470 [i915]
[  120.178081]  i915_gem_do_execbuffer+0x45dc/0x4c20 [i915]
[  120.178292]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
[  120.178309]  drm_ioctl_kernel+0xe4/0x120
[  120.178322]  drm_ioctl+0x297/0x4c7
[  120.178335]  ksys_ioctl+0x89/0xb0
[  120.178348]  __x64_sys_ioctl+0x42/0x60
[  120.178361]  do_syscall_64+0x6e/0x2c0
[  120.178375]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  120.178387]
[  120.178397] read to 0xffff8881e35e6540 of 4 bytes by interrupt on cpu 2:
[  120.178606]  effective_prio+0x25/0xc0 [i915]
[  120.178812]  process_csb+0xe8b/0x10a0 [i915]
[  120.179021]  execlists_submission_tasklet+0x30/0x170 [i915]
[  120.179038]  tasklet_action_common.isra.0+0x42/0xa0
[  120.179053]  __do_softirq+0xd7/0x2cd
[  120.179066]  irq_exit+0xbe/0xe0
[  120.179078]  do_IRQ+0x51/0x100
[  120.179090]  ret_from_intr+0x0/0x1c
[  120.179104]  cpuidle_enter_state+0x1b8/0x5d0
[  120.179117]  cpuidle_enter+0x50/0x90
[  120.179131]  do_idle+0x1a1/0x1f0
[  120.179145]  cpu_startup_entry+0x14/0x16
[  120.179158]  start_secondary+0x120/0x180
[  120.179172]  secondary_startup_64+0xa4/0xb0

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/20200309110934.868-5-chris@chris-wilson.co.uk