Tried talking about this on IRC (#intel-gfx on FreeNode).  It was suggested to me to capture the kernel log after setting drm.debug to 0x5.  That produced quite a lot of log output (70 thousand lines in about a minute), and after some trimming I got something that could perhaps be useful: https://gist.github.com/mgedmin/8686551

This is me unplugging the DisplayPort connector:

Jan 29 13:49:59 platonas kernel: [ 2041.110857] [drm:i915_hotplug_work_func], running encoder hotplug functions
Jan 29 13:49:59 platonas kernel: [ 2041.110861] [drm:i915_hotplug_work_func], Connector HDMI-A-1 (pin 4) received hotplug event.
Jan 29 13:49:59 platonas kernel: [ 2041.110863] [drm:i915_hotplug_work_func], Connector DP-1 (pin 4) received hotplug event.
Jan 29 13:49:59 platonas kernel: [ 2041.125448] [drm:drm_do_probe_ddc_edid], drm: skipping non-existent adapter i915 gmbus dpb
Jan 29 13:49:59 platonas kernel: [ 2041.125457] [drm:intel_hpd_irq_event], [CONNECTOR:16:HDMI-A-1] status updated from 1 to 2
Jan 29 13:49:59 platonas kernel: [ 2041.125464] [drm:intel_hpd_irq_event], [CONNECTOR:20:DP-1] status updated from 2 to 2
Jan 29 13:49:59 platonas kernel: [ 2041.125466] [drm:drm_sysfs_hotplug_event], generating hotplug event

This is me plugging it back in (it wasn't detected by gnome-shell; xrandr --current didn't see it):

Jan 29 13:50:05 platonas kernel: [ 2046.777640] [drm:i915_hotplug_work_func], running encoder hotplug functions
Jan 29 13:50:05 platonas kernel: [ 2046.777655] [drm:i915_hotplug_work_func], Connector HDMI-A-1 (pin 4) received hotplug event.
Jan 29 13:50:05 platonas kernel: [ 2046.777661] [drm:i915_hotplug_work_func], Connector DP-1 (pin 4) received hotplug event.
Jan 29 13:50:05 platonas kernel: [ 2046.778046] [drm:drm_do_probe_ddc_edid], drm: skipping non-existent adapter i915 gmbus dpb
Jan 29 13:50:05 platonas kernel: [ 2046.778051] [drm:intel_hpd_irq_event], [CONNECTOR:16:HDMI-A-1] status updated from 2 to 2
Jan 29 13:50:05 platonas kernel: [ 2046.780633] [drm:intel_dp_aux_ch], dp_aux_ch timeout status 0x7143003f
Jan 29 13:50:05 platonas kernel: [ 2046.789040] [drm:intel_dp_aux_ch], dp_aux_ch timeout status 0x7143003f
Jan 29 13:50:05 platonas kernel: [ 2046.797004] [drm:intel_dp_aux_ch], dp_aux_ch timeout status 0x7143003f
Jan 29 13:50:05 platonas kernel: [ 2046.802468] [drm:intel_hpd_irq_event], [CONNECTOR:20:DP-1] status updated from 2 to 2
Jan 29 13:50:05 platonas kernel: [ 2046.830695] crtc 1 : v 7 p(0,-23)@ 2046.796294 -> 2046.796779 [e 2 us, 0 rep]

This is me running xrandr to force monitor detection:

Jan 29 13:50:13 platonas kernel: [ 2054.453968] [drm:drm_mode_getconnector], [CONNECTOR:7:?]
Jan 29 13:50:13 platonas kernel: [ 2054.453976] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:7:LVDS-1]
Jan 29 13:50:13 platonas kernel: [ 2054.454000] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:7:LVDS-1] probed modes :
Jan 29 13:50:13 platonas kernel: [ 2054.454005] [drm:drm_mode_debug_printmodeline], Modeline 10:"1366x768" 60 74800 1366 1414 1446 1578 768 770 775 790 0x48 0xa
Jan 29 13:50:13 platonas kernel: [ 2054.454022] [drm:drm_mode_getconnector], [CONNECTOR:7:?]
Jan 29 13:50:13 platonas kernel: [ 2054.454571] [drm:drm_mode_getconnector], [CONNECTOR:13:?]
Jan 29 13:50:13 platonas kernel: [ 2054.454576] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:13:VGA-1]
Jan 29 13:50:13 platonas kernel: [ 2054.454583] [drm:intel_ironlake_crt_detect_hotplug], ironlake hotplug adpa=0xf40000, result 0
Jan 29 13:50:13 platonas kernel: [ 2054.454588] [drm:intel_crt_detect], CRT not detected via hotplug
Jan 29 13:50:13 platonas kernel: [ 2054.454766] [drm:gmbus_xfer], GMBUS [i915 gmbus vga] NAK for addr: 0050 r(1)
Jan 29 13:50:13 platonas kernel: [ 2054.454773] [drm:drm_do_probe_ddc_edid], drm: skipping non-existent adapter i915 gmbus vga
Jan 29 13:50:13 platonas kernel: [ 2054.454777] [drm:intel_crt_get_edid], CRT GMBUS EDID read failed, retry using GPIO bit-banging
Jan 29 13:50:13 platonas kernel: [ 2054.454780] [drm:intel_gmbus_force_bit], enabling bit-banging on i915 gmbus vga. force bit now 1
Jan 29 13:50:13 platonas kernel: [ 2054.455160] [drm:drm_do_probe_ddc_edid], drm: skipping non-existent adapter i915 gmbus vga
Jan 29 13:50:13 platonas kernel: [ 2054.455163] [drm:intel_gmbus_force_bit], disabling bit-banging on i915 gmbus vga. force bit now 0
Jan 29 13:50:13 platonas kernel: [ 2054.455167] [drm:intel_crt_detect_ddc], CRT not detected via DDC:0x50 [no valid EDID found]
Jan 29 13:50:13 platonas kernel: [ 2054.455171] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:13:VGA-1] disconnected
Jan 29 13:50:13 platonas kernel: [ 2054.455186] [drm:drm_mode_getconnector], [CONNECTOR:16:?]
Jan 29 13:50:13 platonas kernel: [ 2054.455190] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:16:HDMI-A-1]
Jan 29 13:50:13 platonas kernel: [ 2054.533988] [drm:drm_edid_to_eld], ELD: no CEA Extension found
Jan 29 13:50:13 platonas kernel: [ 2054.534016] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:16:HDMI-A-1] probed modes :
Jan 29 13:50:13 platonas kernel: [ 2054.534021] [drm:drm_mode_debug_printmodeline], Modeline 32:"1280x1024" 60 108000 1280 1328 1440 1688 1024 1025 1028 1066 0x48 0x5
Jan 29 13:50:13 platonas kernel: [ 2054.534028] [drm:drm_mode_debug_printmodeline], Modeline 42:"1280x1024" 75 135000 1280 1296 1440 1688 1024 1025 1028 1066 0x40 0x5
Jan 29 13:50:13 platonas kernel: [ 2054.534034] [drm:drm_mode_debug_printmodeline], Modeline 33:"1280x960" 60 108000 1280 1376 1488 1800 960 961 964 1000 0x40 0x5
Jan 29 13:50:13 platonas kernel: [ 2054.534040] [drm:drm_mode_debug_printmodeline], Modeline 34:"1152x864" 75 108000 1152 1216 1344 1600 864 865 868 900 0x40 0x5
Jan 29 13:50:13 platonas kernel: [ 2054.534046] [drm:drm_mode_debug_printmodeline], Modeline 43:"1024x768" 75 78800 1024 1040 1136 1312 768 769 772 800 0x40 0x5
Jan 29 13:50:13 platonas kernel: [ 2054.534052] [drm:drm_mode_debug_printmodeline], Modeline 44:"1024x768" 70 75000 1024 1048 1184 1328 768 771 777 806 0x40 0xa
Jan 29 13:50:13 platonas kernel: [ 2054.534064] [drm:drm_mode_debug_printmodeline], Modeline 45:"1024x768" 60 65000 1024 1048 1184 1344 768 771 777 806 0x40 0xa
Jan 29 13:50:13 platonas kernel: [ 2054.534065] [drm:drm_mode_debug_printmodeline], Modeline 46:"832x624" 75 57284 832 864 928 1152 624 625 628 667 0x40 0xa
Jan 29 13:50:13 platonas kernel: [ 2054.534067] [drm:drm_mode_debug_printmodeline], Modeline 51:"800x600" 75 49500 800 816 896 1056 600 601 604 625 0x40 0x5
Jan 29 13:50:13 platonas kernel: [ 2054.534068] [drm:drm_mode_debug_printmodeline], Modeline 52:"800x600" 72 50000 800 856 976 1040 600 637 643 666 0x40 0x5
Jan 29 13:50:13 platonas kernel: [ 2054.534070] [drm:drm_mode_debug_printmodeline], Modeline 35:"800x600" 60 40000 800 840 968 1056 600 601 605 628 0x40 0x5
Jan 29 13:50:13 platonas kernel: [ 2054.534072] [drm:drm_mode_debug_printmodeline], Modeline 36:"800x600" 56 36000 800 824 896 1024 600 601 603 625 0x40 0x5
Jan 29 13:50:13 platonas kernel: [ 2054.534073] [drm:drm_mode_debug_printmodeline], Modeline 37:"640x480" 75 31500 640 656 720 840 480 481 484 500 0x40 0xa
Jan 29 13:50:13 platonas kernel: [ 2054.534075] [drm:drm_mode_debug_printmodeline], Modeline 38:"640x480" 73 31500 640 664 704 832 480 489 491 520 0x40 0xa
Jan 29 13:50:13 platonas kernel: [ 2054.534076] [drm:drm_mode_debug_printmodeline], Modeline 39:"640x480" 67 30240 640 704 768 864 480 483 486 525 0x40 0xa
Jan 29 13:50:13 platonas kernel: [ 2054.534078] [drm:drm_mode_debug_printmodeline], Modeline 40:"640x480" 60 25200 640 656 752 800 480 490 492 525 0x40 0xa
Jan 29 13:50:13 platonas kernel: [ 2054.534079] [drm:drm_mode_debug_printmodeline], Modeline 41:"720x400" 70 28320 720 738 846 900 400 412 414 449 0x40 0x6
Jan 29 13:50:13 platonas kernel: [ 2054.534114] [drm:drm_mode_getconnector], [CONNECTOR:16:?]
Jan 29 13:50:13 platonas kernel: [ 2054.534149] [drm:drm_mode_getconnector], [CONNECTOR:20:?]
Jan 29 13:50:13 platonas kernel: [ 2054.534151] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:20:DP-1]
Jan 29 13:50:13 platonas kernel: [ 2054.536673] [drm:intel_dp_aux_ch], dp_aux_ch timeout status 0x7143003f
Jan 29 13:50:13 platonas kernel: [ 2054.546230] [drm:intel_dp_aux_ch], dp_aux_ch timeout status 0x7143003f
Jan 29 13:50:13 platonas kernel: [ 2054.554225] [drm:intel_dp_aux_ch], dp_aux_ch timeout status 0x7143003f
Jan 29 13:50:13 platonas kernel: [ 2054.559723] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:20:DP-1] disconnected
Jan 29 13:50:13 platonas kernel: [ 2054.559768] [drm:drm_mode_getconnector], [CONNECTOR:22:?]
Jan 29 13:50:13 platonas kernel: [ 2054.559773] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:22:HDMI-A-2]
Jan 29 13:50:13 platonas kernel: [ 2054.560047] [drm:gmbus_xfer], GMBUS [i915 gmbus dpc] NAK for addr: 0050 r(1)
Jan 29 13:50:13 platonas kernel: [ 2054.560060] [drm:drm_do_probe_ddc_edid], drm: skipping non-existent adapter i915 gmbus dpc
Jan 29 13:50:13 platonas kernel: [ 2054.560066] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:22:HDMI-A-2] disconnected
Jan 29 13:50:13 platonas kernel: [ 2054.560089] [drm:drm_mode_getconnector], [CONNECTOR:24:?]
Jan 29 13:50:13 platonas kernel: [ 2054.560095] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:24:HDMI-A-3]
Jan 29 13:50:13 platonas kernel: [ 2054.560357] [drm:gmbus_xfer], GMBUS [i915 gmbus dpd] NAK for addr: 0050 r(1)
Jan 29 13:50:13 platonas kernel: [ 2054.560370] [drm:drm_do_probe_ddc_edid], drm: skipping non-existent adapter i915 gmbus dpd
Jan 29 13:50:13 platonas kernel: [ 2054.560376] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:24:HDMI-A-3] disconnected
Jan 29 13:50:13 platonas kernel: [ 2054.560399] [drm:drm_mode_getconnector], [CONNECTOR:26:?]
Jan 29 13:50:13 platonas kernel: [ 2054.560405] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:26:DP-2]
Jan 29 13:50:13 platonas kernel: [ 2054.560412] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:26:DP-2] disconnected
Jan 29 13:50:13 platonas kernel: [ 2054.560421] [drm:drm_mode_getconnector], [CONNECTOR:28:?]
Jan 29 13:50:13 platonas kernel: [ 2054.560425] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:28:DP-3]
Jan 29 13:50:13 platonas kernel: [ 2054.560431] [drm:drm_helper_probe_single_connector_modes], [CONNECTOR:28:DP-3] disconnected