I have the same on an Intel NUC6i5SYK on ubuntu 16.04 with kernel 4.4.0-21-generic #37-Ubuntu SMP Mon Apr 18 18:33:37 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux. Two identical DVI monitors are attached, one via HDMI, one via Displayport:

[    3.216454] ------------[ cut here ]------------
[    3.216472] WARNING: CPU: 0 PID: 41 at /build/linux-Ay7j_C/linux-4.4.0/ubuntu/i915/intel_pm.c:3572 skl_update_other_pipe_wm+0x16c/0
x180 [i915_bpo]()
[    3.216473] WARN_ON(!wm_changed)
[    3.216474] Modules linked in:
[    3.216475]  hid_generic(OE) usbhid(OE) i915_bpo intel_ips i2c_algo_bit drm_kms_helper syscopyarea e1000e sysfillrect sysimgblt ahc
i fb_sys_fops libahci drm sdhci_pci ptp pps_core sdhci video pinctrl_sunrisepoint pinctrl_intel i2c_hid(OE) hid(OE) fjes
[    3.216486] CPU: 0 PID: 41 Comm: kworker/0:1 Tainted: G           OE   4.4.0-21-generic #37-Ubuntu
[    3.216487] Hardware name:                  /NUC6i5SYB, BIOS SYSKLi35.86A.0042.2016.0409.1246 04/09/2016
[    3.216492] Workqueue: events output_poll_execute [drm_kms_helper]
[    3.216494]  0000000000000286 0000000002de39f2 ffff88046bacf950 ffffffff813e93c3
[    3.216495]  ffff88046bacf998 ffffffffc029bc70 ffff88046bacf988 ffffffff81080f62
[    3.216497]  ffff880465400000 ffff880465609d9c ffff880465401000 ffff8804662dd378
[    3.216498] Call Trace:
[    3.216502]  [<ffffffff813e93c3>] dump_stack+0x63/0x90
[    3.216504]  [<ffffffff81080f62>] warn_slowpath_common+0x82/0xc0
[    3.216506]  [<ffffffff81080ffc>] warn_slowpath_fmt+0x5c/0x80
[    3.216516]  [<ffffffffc01cd5cc>] skl_update_other_pipe_wm+0x16c/0x180 [i915_bpo]
[    3.216526]  [<ffffffffc01cd766>] skl_update_wm+0x186/0x5f0 [i915_bpo]
[    3.216542]  [<ffffffffc025936f>] ? intel_ddi_enable_transcoder_func+0x17f/0x260 [i915_bpo]
[    3.216552]  [<ffffffffc01d148e>] intel_update_watermarks+0x1e/0x30 [i915_bpo]
[    3.216567]  [<ffffffffc023d401>] haswell_crtc_enable+0x321/0x8c0 [i915_bpo]
[    3.216582]  [<ffffffffc0229a8e>] ? intel_finish_crtc_commit+0xe/0x10 [i915_bpo]
[    3.216586]  [<ffffffffc0185324>] ? drm_atomic_helper_commit_planes_on_crtc+0x154/0x270 [drm_kms_helper]
[    3.216601]  [<ffffffffc0239dad>] intel_atomic_commit+0x5dd/0xdb0 [i915_bpo]
[    3.216614]  [<ffffffffc00a480e>] ? drm_atomic_check_only+0x18e/0x590 [drm]
[    3.216624]  [<ffffffffc00a4c47>] drm_atomic_commit+0x37/0x60 [drm]
[    3.216629]  [<ffffffffc01865ef>] restore_fbdev_mode+0x22f/0x260 [drm_kms_helper]
[    3.216638]  [<ffffffffc00a346a>] ? drm_modeset_lock_all_ctx+0x9a/0xb0 [drm]
[    3.216643]  [<ffffffffc01887d3>] drm_fb_helper_restore_fbdev_mode_unlocked+0x33/0x80 [drm_kms_helper]
[    3.216647]  [<ffffffffc018884d>] drm_fb_helper_set_par+0x2d/0x50 [drm_kms_helper]
[    3.216651]  [<ffffffffc0188752>] drm_fb_helper_hotplug_event+0xd2/0x120 [drm_kms_helper]
[    3.216666]  [<ffffffffc025065e>] intel_fbdev_output_poll_changed+0x1e/0x30 [i915_bpo]
[    3.216669]  [<ffffffffc017b6f7>] drm_kms_helper_hotplug_event+0x27/0x30 [drm_kms_helper]
[    3.216672]  [<ffffffffc017b8f7>] output_poll_execute+0x197/0x1e0 [drm_kms_helper]
[    3.216675]  [<ffffffff81099ff2>] process_one_work+0x162/0x480
[    3.216676]  [<ffffffff8109a35b>] worker_thread+0x4b/0x4c0
[    3.216678]  [<ffffffff8109a310>] ? process_one_work+0x480/0x480
[    3.216679]  [<ffffffff8109a310>] ? process_one_work+0x480/0x480
[    3.216681]  [<ffffffff810a0528>] kthread+0xd8/0xf0
[    3.216682]  [<ffffffff810a0450>] ? kthread_create_on_node+0x1e0/0x1e0
[    3.216685]  [<ffffffff8182488f>] ret_from_fork+0x3f/0x70
[    3.216686]  [<ffffffff810a0450>] ? kthread_create_on_node+0x1e0/0x1e0
[    3.216687] ---[ end trace 59d8412037f5e9ed ]---

Should I do apport-collect 1567417 or does this confuse this bug report?