drm/i915/gt: Mark up racy check of last list element

[   25.025543] BUG: KCSAN: data-race in __i915_request_create [i915] / process_csb [i915]
[   25.025561]
[   25.025573] write (marked) to 0xffff8881e85c1620 of 8 bytes by task 696 on cpu 1:
[   25.025789]  __i915_request_create+0x54b/0x5d0 [i915]
[   25.026001]  i915_request_create+0xcc/0x150 [i915]
[   25.026218]  i915_gem_do_execbuffer+0x2f70/0x4c20 [i915]
[   25.026428]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
[   25.026445]  drm_ioctl_kernel+0xe4/0x120
[   25.026459]  drm_ioctl+0x297/0x4c7
[   25.026472]  ksys_ioctl+0x89/0xb0
[   25.026484]  __x64_sys_ioctl+0x42/0x60
[   25.026497]  do_syscall_64+0x6e/0x2c0
[   25.026510]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   25.026522]
[   25.026532] read to 0xffff8881e85c1620 of 8 bytes by interrupt on cpu 2:
[   25.026742]  process_csb+0x8d6/0x1070 [i915]
[   25.026949]  execlists_submission_tasklet+0x30/0x170 [i915]
[   25.026969]  tasklet_action_common.isra.0+0x42/0xa0
[   25.026984]  __do_softirq+0xd7/0x2cd
[   25.026997]  irq_exit+0xbe/0xe0
[   25.027009]  do_IRQ+0x51/0x100
[   25.027021]  ret_from_intr+0x0/0x1c
[   25.027033]  poll_idle+0x3e/0x13b
[   25.027047]  cpuidle_enter_state+0x189/0x5d0
[   25.027060]  cpuidle_enter+0x50/0x90
[   25.027074]  do_idle+0x1a1/0x1f0
[   25.027086]  cpu_startup_entry+0x14/0x16
[   25.027100]  start_secondary+0x120/0x180
[   25.027116]  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/20200309110934.868-2-chris@chris-wilson.co.uk