Description of problem:
The warning is reasonably accurate.  I connected both DisplayPort and HDMI to the same mini-DisplayPort on my XPS 13 (2015 version) laptop, using an adapter that provides both.

I think this warning should be gracefully handled/ignored. I don't expect both ports to work in this situation. I didn't even expect the HDMI port to work at all, since the logo on the XPS 13 does not specify DisplayPort++.

Additional info:
reporter:       libreport-2.6.2
WARNING: CPU: 0 PID: 2921 at drivers/gpu/drm/i915/intel_hdmi.c:56 assert_hdmi_port_disabled+0x4a/0x50 [i915]()
HDMI port enabled, expecting disabled
Modules linked in: macvlan nfnetlink_queue nfnetlink_log nfnetlink asix usbnet mii rfcomm fuse ccm cmac xt_CHECKSUM ipt_MASQUERADE nf_nat_masquerade_ipv4 tun nf_conntrack_netbios_ns nf_conntrack_broadcast ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw bnep arc4 iTCO_wdt iwlmvm iTCO_vendor_support intel_rapl dell_wmi sparse_keymap iosf_mbi x86_pkg_temp_thermal mac80211 coretemp kvm_intel vfat fat snd_hda_codec_hdmi kvm dell_led iwlwifi snd_hda_codec_realtek snd_hda_codec_generic
 dell_laptop dcdbas uvcvideo snd_hda_intel snd_hda_controller videobuf2_vmalloc videobuf2_core snd_hda_codec snd_soc_rt286 snd_soc_core snd_compress cfg80211 snd_pcm_dmaengine snd_hda_core snd_hwdep snd_seq videobuf2_memops v4l2_common snd_seq_device videodev btusb btbcm snd_pcm rtsx_pci_ms i2c_i801 btintel media memstick bluetooth snd_timer lpc_ich snd mei_me shpchp rfkill soundcore wmi mei i2c_designware_platform soc_button_array dw_dmac int3403_thermal snd_soc_sst_acpi dw_dmac_core i2c_designware_core int3400_thermal tpm_crb int3402_thermal acpi_thermal_rel processor_thermal_device tpm int340x_thermal_zone acpi_pad nfsd auth_rpcgss nfs_acl lockd grace sunrpc binfmt_misc dm_crypt hid_multitouch i915 i2c_algo_bit drm_kms_helper rtsx_pci_sdmmc crct10dif_pclmul crc32_pclmul crc32c_intel
 drm ghash_clmulni_intel serio_raw rtsx_pci mfd_core sdhci_acpi sdhci mmc_core video i2c_hid
CPU: 0 PID: 2921 Comm: Xorg Not tainted 4.1.7-200.fc22.x86_64 #1
Hardware name: Dell Inc. XPS 13 9343/09K8G1, BIOS A05 07/14/2015
 0000000000000000 00000000a138469e ffff8800cd4af8e8 ffffffff8179972d
 0000000000000000 ffff8800cd4af940 ffff8800cd4af928 ffffffff810a165a
 ffff8802151a1190 ffff880212f70000 ffff8800d9644000 0000000000060200
Call Trace:
 [<ffffffff8179972d>] dump_stack+0x45/0x57
 [<ffffffff810a165a>] warn_slowpath_common+0x8a/0xc0
 [<ffffffff810a16e5>] warn_slowpath_fmt+0x55/0x70
 [<ffffffffa01be07a>] assert_hdmi_port_disabled+0x4a/0x50 [i915]
 [<ffffffffa01be53a>] hsw_set_infoframes+0x6a/0x100 [i915]
 [<ffffffffa01a9ff8>] intel_ddi_pre_enable+0x1e8/0x2f0 [i915]
 [<ffffffffa018ff8f>] haswell_crtc_enable+0x41f/0xb50 [i915]
 [<ffffffffa018cd92>] __intel_set_mode+0xaa2/0xce0 [i915]
 [<ffffffffa00af43e>] ? drm_atomic_get_crtc_state+0x1e/0xc0 [drm]
 [<ffffffffa01945ff>] intel_crtc_set_config+0xd3f/0x1080 [i915]
 [<ffffffffa009f516>] drm_mode_set_config_internal+0x66/0x100 [drm]
 [<ffffffffa00a3866>] drm_mode_setcrtc+0x366/0x650 [drm]
 [<ffffffffa00939df>] drm_ioctl+0x12f/0x620 [drm]
 [<ffffffffa00a3500>] ? drm_mode_setplane+0x1d0/0x1d0 [drm]
 [<ffffffff810136d6>] ? __switch_to+0x216/0x5d0
 [<ffffffff8123fe06>] do_vfs_ioctl+0x2c6/0x4d0
 [<ffffffff81240091>] SyS_ioctl+0x81/0xa0
 [<ffffffff8179fcee>] system_call_fastpath+0x12/0x71