On entering sleep mode, I get the following warning:

kernel: ------------[ cut here ]------------
kernel: WARNING: CPU: 0 PID: 3076 at drivers/gpu/drm/i915/intel_display.c:7816 hsw_enable_pc8+0x599/0x620 [i915]()
kernel: Power well on
kernel: Modules linked in: fuse ctr ccm rfcomm ecb af_packet bnep btusb bluetooth iwlmvm arc4 rtl8723be btcoexist rtl8723_common rtl_pci rtl
kernel: CPU: 0 PID: 3076 Comm: kworker/u16:4 Not tainted 3.19.0-wd-wl+ #44
kernel: Hardware name: TOSHIBA TECRA A50-A/TECRA A50-A, BIOS Version 4.20   04/17/2014
kernel: Workqueue: events_unbound async_run_entry_fn
kernel:  ffffffffa0608b70 ffff88021854bb58 ffffffff816cb40a 0000000000006832
kernel:  ffff88021854bba8 ffff88021854bb98 ffffffff8105db8a ffff88021854bb78
kernel:  ffff8802186b0000 ffff8800c7a037a0 ffff8800c7a037b0 ffff8800c7a03000
kernel: Call Trace:
kernel:  [<ffffffff816cb40a>] dump_stack+0x4c/0x6e
kernel:  [<ffffffff8105db8a>] warn_slowpath_common+0x8a/0xc0
kernel:  [<ffffffff8105dc06>] warn_slowpath_fmt+0x46/0x50
kernel:  [<ffffffffa05b37f9>] hsw_enable_pc8+0x599/0x620 [i915]
kernel:  [<ffffffffa0547760>] ? i915_suspend_legacy+0xb0/0xb0 [i915]
kernel:  [<ffffffffa054708b>] intel_suspend_complete+0x10b/0x6d0 [i915]
kernel:  [<ffffffffa0547668>] i915_drm_suspend_late+0x18/0x60 [i915]
kernel:  [<ffffffffa0547780>] i915_pm_suspend_late+0x20/0x30 [i915]
kernel:  [<ffffffff8149e88d>] dpm_run_callback+0x6d/0x1d0
kernel:  [<ffffffff8149f310>] __device_suspend_late+0xb0/0x1a0
kernel:  [<ffffffff8149f41f>] async_suspend_late+0x1f/0xa0
kernel:  [<ffffffff8108004c>] async_run_entry_fn+0x4c/0x160
kernel:  [<ffffffff8107778c>] process_one_work+0x1cc/0x540
kernel:  [<ffffffff81077711>] ? process_one_work+0x151/0x540
kernel:  [<ffffffff8137f41f>] ? __delay+0xf/0x20
kernel:  [<ffffffff81077c21>] worker_thread+0x121/0x460
kernel:  [<ffffffff81077b00>] ? process_one_work+0x540/0x540
kernel:  [<ffffffff8107d1af>] kthread+0xef/0x110
kernel:  [<ffffffff8107d0c0>] ? kthread_create_on_node+0x220/0x220
kernel:  [<ffffffff816d48ec>] ret_from_fork+0x7c/0xb0
kernel:  [<ffffffff8107d0c0>] ? kthread_create_on_node+0x220/0x220
kernel: ---[ end trace b71420f35eebee99 ]---

This bug is the same as https://bugzilla.redhat.com/show_bug.cgi?id=1167377.