Comment 39 for bug 1723619

This still exists on 4.17-rc3 with my GTX 980 (GM204).
Reverting 13a86519202c5d119d83640d6f781f3181205d2c gets my DP monitor working again. (My HDMI monitor works in both cases)

The DP monitor works (low-resolution console) until nouveau is loaded, then it goes blank.

dmesg output on stock 4.17-rc3 source:
[ 6.790387] fb: switching to nouveaufb from VESA VGA
[ 6.800232] input: HDA Intel PCH Front Mic as /devices/pci0000:00/0000:00:1b.0/sound/card0/input11
[ 6.828574] Console: switching to colour dummy device 80x25
[ 6.828726] nouveau 0000:01:00.0: NVIDIA GM204 (124000a1)
[ 6.829091] input: HDA Intel PCH Rear Mic as /devices/pci0000:00/0000:00:1b.0/sound/card0/input12
[ 6.829150] input: HDA Intel PCH Line as /devices/pci0000:00/0000:00:1b.0/sound/card0/input13
[ 6.829192] input: HDA Intel PCH Line Out Front as /devices/pci0000:00/0000:00:1b.0/sound/card0/input14
[ 6.829235] input: HDA Intel PCH Line Out Surround as /devices/pci0000:00/0000:00:1b.0/sound/card0/input15
[ 6.829276] input: HDA Intel PCH Line Out CLFE as /devices/pci0000:00/0000:00:1b.0/sound/card0/input16
[ 6.829322] input: HDA Intel PCH Line Out Side as /devices/pci0000:00/0000:00:1b.0/sound/card0/input17
[ 6.829369] input: HDA Intel PCH Front Headphone as /devices/pci0000:00/0000:00:1b.0/sound/card0/input18
[ 6.841234] fuse init (API version 7.26)
[ 6.888971] EXT4-fs (sdb1): mounted filesystem with ordered data mode. Opts: data=ordered
[ 6.911531] nouveau 0000:01:00.0: bios: version 84.04.2f.00.4b
[ 6.913746] nouveau 0000:01:00.0: fb: 4096 MiB GDDR5
[ 6.913762] nouveau 0000:01:00.0: bus: MMIO write of 8000012c FAULT at 10eb14 [ IBUS ]
[ 6.976881] [TTM] Zone kernel: Available graphics memory: 8185092 kiB
[ 6.976886] [TTM] Zone dma32: Available graphics memory: 2097152 kiB
[ 6.976889] [TTM] Initializing pool allocator
[ 6.976892] [TTM] Initializing DMA pool allocator
[ 6.976902] nouveau 0000:01:00.0: DRM: VRAM: 4096 MiB
[ 6.976904] nouveau 0000:01:00.0: DRM: GART: 1048576 MiB
[ 6.976907] nouveau 0000:01:00.0: DRM: TMDS table version 2.0
[ 6.976909] nouveau 0000:01:00.0: DRM: DCB version 4.1
[ 6.976911] nouveau 0000:01:00.0: DRM: DCB outp 00: 01000f02 00020030
[ 6.976913] nouveau 0000:01:00.0: DRM: DCB outp 01: 02000f00 00000000
[ 6.976915] nouveau 0000:01:00.0: DRM: DCB outp 02: 04811f96 04400020
[ 6.976916] nouveau 0000:01:00.0: DRM: DCB outp 03: 04011f92 00020020
[ 6.976918] nouveau 0000:01:00.0: DRM: DCB outp 04: 02822f76 04400020
[ 6.976920] nouveau 0000:01:00.0: DRM: DCB outp 05: 02022f72 00020020
[ 6.976922] nouveau 0000:01:00.0: DRM: DCB outp 06: 02033f62 00020010
[ 6.976923] nouveau 0000:01:00.0: DRM: DCB outp 07: 04844f86 04400010
[ 6.976925] nouveau 0000:01:00.0: DRM: DCB outp 08: 04044f82 00020010
[ 6.976927] nouveau 0000:01:00.0: DRM: DCB outp 15: 01df6ff8 00000000
[ 6.976929] nouveau 0000:01:00.0: DRM: DCB conn 00: 00001030
[ 6.976930] nouveau 0000:01:00.0: DRM: DCB conn 01: 02000146
[ 6.976932] nouveau 0000:01:00.0: DRM: DCB conn 02: 00020246
[ 6.976933] nouveau 0000:01:00.0: DRM: DCB conn 03: 00010361
[ 6.976935] nouveau 0000:01:00.0: DRM: DCB conn 04: 01000446
[ 6.976937] nouveau 0000:01:00.0: DRM: DCB conn 06: 00000670
[ 7.018129] nouveau 0000:01:00.0: DRM: failed to create encoder 1/8/0: -19
[ 7.018134] nouveau 0000:01:00.0: DRM: Virtual-1 has no encoders, removing
[ 7.018162] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[ 7.018165] [drm] Driver supports precise vblank timestamp query.
[ 7.122492] nouveau 0000:01:00.0: DRM: MM: using COPY for buffer copies
[ 7.138298] nouveau 0000:01:00.0: DRM: DDC responded, but no EDID for DP-1
[ 7.230500] input: HDA NVidia HDMI/DP,pcm=3 as /devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input19
[ 7.230532] input: HDA NVidia HDMI/DP,pcm=7 as /devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input20
[ 7.230556] input: HDA NVidia HDMI/DP,pcm=8 as /devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input21
[ 7.230579] input: HDA NVidia HDMI/DP,pcm=9 as /devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input22
[ 7.402595] nouveau 0000:01:00.0: DRM: allocated 1920x1080 fb: 0x80000, bo (ptrval)
[ 7.402642] fbcon: nouveaufb (fb0) is primary device
[ 7.417391] nouveau 0000:01:00.0: DRM: DDC responded, but no EDID for DP-1
[ 7.650543] nouveau 0000:01:00.0: DRM: DDC responded, but no EDID for DP-1
[ 7.835686] Console: switching to colour frame buffer device 240x67
[ 7.869512] nouveau 0000:01:00.0: fb0: nouveaufb frame buffer device
[ 7.906730] [drm] Initialized nouveau 1.3.1 20120801 for 0000:01:00.0 on minor 0
[ 9.137286] nouveau 0000:01:00.0: DRM: DDC responded, but no EDID for DP-1
[ 16.237674] nouveau 0000:01:00.0: DRM: DDC responded, but no EDID for DP-1

dmesg with the regression-inducing commit reverted:
[ 7.893930] fb: switching to nouveaufb from VESA VGA
[ 8.082202] Console: switching to colour dummy device 80x25
[ 8.082527] nouveau 0000:01:00.0: NVIDIA GM204 (124000a1)
[ 8.104913] fuse init (API version 7.26)
[ 8.105261] intel_rapl: Found RAPL domain package
[ 8.105265] intel_rapl: Found RAPL domain core
[ 8.105267] intel_rapl: Found RAPL domain dram
[ 8.166362] nouveau 0000:01:00.0: bios: version 84.04.2f.00.4b
[ 8.168851] nouveau 0000:01:00.0: fb: 4096 MiB GDDR5
[ 8.168875] nouveau 0000:01:00.0: bus: MMIO read of 00000000 FAULT at 022554 [ IBUS ]
[ 8.227132] EXT4-fs (sdb1): mounted filesystem with ordered data mode. Opts: data=ordered
[ 8.231196] [TTM] Zone kernel: Available graphics memory: 8185088 kiB
[ 8.231199] [TTM] Zone dma32: Available graphics memory: 2097152 kiB
[ 8.231201] [TTM] Initializing pool allocator
[ 8.231204] [TTM] Initializing DMA pool allocator
[ 8.231213] nouveau 0000:01:00.0: DRM: VRAM: 4096 MiB
[ 8.231215] nouveau 0000:01:00.0: DRM: GART: 1048576 MiB
[ 8.231218] nouveau 0000:01:00.0: DRM: TMDS table version 2.0
[ 8.231219] nouveau 0000:01:00.0: DRM: DCB version 4.1
[ 8.231221] nouveau 0000:01:00.0: DRM: DCB outp 00: 01000f02 00020030
[ 8.231223] nouveau 0000:01:00.0: DRM: DCB outp 01: 02000f00 00000000
[ 8.231225] nouveau 0000:01:00.0: DRM: DCB outp 02: 04811f96 04400020
[ 8.231227] nouveau 0000:01:00.0: DRM: DCB outp 03: 04011f92 00020020
[ 8.231229] nouveau 0000:01:00.0: DRM: DCB outp 04: 02822f76 04400020
[ 8.231231] nouveau 0000:01:00.0: DRM: DCB outp 05: 02022f72 00020020
[ 8.231232] nouveau 0000:01:00.0: DRM: DCB outp 06: 02033f62 00020010
[ 8.231234] nouveau 0000:01:00.0: DRM: DCB outp 07: 04844f86 04400010
[ 8.231236] nouveau 0000:01:00.0: DRM: DCB outp 08: 04044f82 00020010
[ 8.231238] nouveau 0000:01:00.0: DRM: DCB outp 15: 01df6ff8 00000000
[ 8.231239] nouveau 0000:01:00.0: DRM: DCB conn 00: 00001030
[ 8.231241] nouveau 0000:01:00.0: DRM: DCB conn 01: 02000146
[ 8.231243] nouveau 0000:01:00.0: DRM: DCB conn 02: 00020246
[ 8.231244] nouveau 0000:01:00.0: DRM: DCB conn 03: 00010361
[ 8.231246] nouveau 0000:01:00.0: DRM: DCB conn 04: 01000446
[ 8.231247] nouveau 0000:01:00.0: DRM: DCB conn 06: 00000670
[ 8.272499] nouveau 0000:01:00.0: DRM: failed to create encoder 1/8/0: -19
[ 8.272507] nouveau 0000:01:00.0: DRM: Virtual-1 has no encoders, removing
[ 8.272552] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[ 8.272557] [drm] Driver supports precise vblank timestamp query.
[ 8.371715] nouveau 0000:01:00.0: DRM: MM: using COPY for buffer copies
[ 8.550449] input: HDA NVidia HDMI/DP,pcm=3 as /devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input19
[ 8.550480] input: HDA NVidia HDMI/DP,pcm=7 as /devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input20
[ 8.550506] input: HDA NVidia HDMI/DP,pcm=8 as /devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input21
[ 8.550531] input: HDA NVidia HDMI/DP,pcm=9 as /devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input22
[ 8.655960] nouveau 0000:01:00.0: DRM: allocated 2560x1440 fb: 0x80000, bo (ptrval)
[ 8.656719] fbcon: nouveaufb (fb0) is primary device
[ 8.912908] Console: switching to colour frame buffer device 240x67
[ 9.130400] nouveau 0000:01:00.0: fb0: nouveaufb frame buffer device
[ 9.160063] [drm] Initialized nouveau 1.3.1 20120801 for 0000:01:00.0 on minor 0

Full dmesg captures are available at https://gist.github.com/aswild/d8fe1b9700c60452b4c9ea79b419d4ee