This prevents a deadlock that somehow results from the suspend() ->
forbid() -> resume() callchain.

[  125.266960] [drm] Initialized nouveau 1.3.1 20120801 for 0000:02:00.0 on minor 1
[  370.120872] INFO: task kworker/4:1:77 blocked for more than 120 seconds.
[  370.120920]       Tainted: G           O    4.12.0-rc3 #20
[  370.120947] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  370.120982] kworker/4:1     D13808    77      2 0x00000000
[  370.120998] Workqueue: pm pm_runtime_work
[  370.121004] Call Trace:
[  370.121018]  __schedule+0x2bf/0xb40
[  370.121025]  ? mark_held_locks+0x5f/0x90
[  370.121038]  schedule+0x3d/0x90
[  370.121044]  rpm_resume+0x107/0x870
[  370.121052]  ? finish_wait+0x90/0x90
[  370.121065]  ? pci_pm_runtime_resume+0xa0/0xa0
[  370.121070]  pm_runtime_forbid+0x4c/0x60
[  370.121129]  nouveau_pmops_runtime_suspend+0xaf/0xc0 [nouveau]
[  370.121139]  pci_pm_runtime_suspend+0x5f/0x170
[  370.121147]  ? pci_pm_runtime_resume+0xa0/0xa0
[  370.121152]  __rpm_callback+0xb9/0x1e0
[  370.121159]  ? pci_pm_runtime_resume+0xa0/0xa0
[  370.121166]  rpm_callback+0x24/0x80
[  370.121171]  ? pci_pm_runtime_resume+0xa0/0xa0
[  370.121176]  rpm_suspend+0x138/0x6e0
[  370.121192]  pm_runtime_work+0x7b/0xc0
[  370.121199]  process_one_work+0x253/0x6a0
[  370.121216]  worker_thread+0x4d/0x3b0
[  370.121229]  kthread+0x133/0x150
[  370.121234]  ? process_one_work+0x6a0/0x6a0
[  370.121238]  ? kthread_create_on_node+0x70/0x70
[  370.121246]  ret_from_fork+0x2a/0x40
[  370.121283]
               Showing all locks held in the system:
[  370.121291] 2 locks held by kworker/4:1/77:
[  370.121298]  #0:  ("pm"){.+.+.+}, at: [<ffffffffac0d3530>] process_one_work+0x1d0/0x6a0
[  370.121315]  #1:  ((&dev->power.work)){+.+.+.}, at: [<ffffffffac0d3530>] process_one_work+0x1d0/0x6a0
[  370.121330] 1 lock held by khungtaskd/81:
[  370.121333]  #0:  (tasklist_lock){.+.+..}, at: [<ffffffffac10fc8d>] debug_show_all_locks+0x3d/0x1a0
[  370.121355] 1 lock held by dmesg/1639:
[  370.121358]  #0:  (&user->lock){+.+.+.}, at: [<ffffffffac124b6d>] devkmsg_read+0x4d/0x360

[  370.121377] =============================================
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>