I can confirm the following:

I had to revert to: 4.13.16-041316-generic and it works great with this kernel !
I'll stick with this kernel until the problem gets solved.

However, from 4.15.* it seems like it doesn't work (with the kernels I tested).

I also tested with "linux-image-unsigned-4.18.5-041805-generic" (upstream) and it still doesn't work, and when I installed it, it complains about "possible not finding firmware /lib/nvidia.... for nouveau". (see below)

Code:
update-initramfs: Generating /boot/initrd.img-4.18.5-041805-generic
W: Possible missing firmware /lib/firmware/nvidia/gv100/sec2/sig.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/sec2/image.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/sec2/desc.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/nvdec/scrubber.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/sw_method_init.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/sw_bundle_init.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/sw_nonctx.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/sw_ctx.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/gpccs_sig.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/gpccs_data.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/gpccs_inst.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/gpccs_bl.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/fecs_sig.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/fecs_data.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/fecs_inst.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/gr/fecs_bl.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/acr/ucode_unload.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/acr/ucode_load.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/acr/unload_bl.bin for module nouveau
W: Possible missing firmware /lib/firmware/nvidia/gv100/acr/bl.bin for module nouveau

DMESG from this kernel:

[    3.123458] nouveau 0000:02:00.0: enabling device (0006 -> 0007)
[    3.124175] fb: switching to nouveaufb from EFI VGA
[    3.124515] nouveau 0000:02:00.0: NVIDIA MCP89 (0af100a2)
[    3.140564] nouveau 0000:02:00.0: bios: version 70.89.13.00.00
[    3.163102] nouveau 0000:02:00.0: fb: 256 MiB stolen system memory
[    3.329183] nouveau 0000:02:00.0: bus: MMIO write of 0000807e FAULT at 100c1c
[    4.668287] nouveau 0000:02:00.0: DRM: VRAM: 256 MiB
[    4.668290] nouveau 0000:02:00.0: DRM: GART: 1048576 MiB
[    4.668297] nouveau 0000:02:00.0: DRM: TMDS table version 2.0
[    4.668300] nouveau 0000:02:00.0: DRM: DCB version 4.0
[    4.668305] nouveau 0000:02:00.0: DRM: DCB outp 00: 040001b6 0f220010
[    4.668309] nouveau 0000:02:00.0: DRM: DCB outp 01: 020112a6 0f220010
[    4.668312] nouveau 0000:02:00.0: DRM: DCB outp 02: 02011262 00020010
[    4.668316] nouveau 0000:02:00.0: DRM: DCB conn 00: 00002047
[    4.668319] nouveau 0000:02:00.0: DRM: DCB conn 01: 00101146
[    4.705380] nouveau 0000:02:00.0: DRM: MM: using M2MF for buffer copies
[    4.801272] nouveau 0000:02:00.0: DRM: allocated 1920x1080 fb: 0x50000, bo (____ptrval____)
[    4.802060] fbcon: nouveaufb (fb0) is primary device
[    4.920114] nouveau 0000:02:00.0: fb0: nouveaufb frame buffer device
[    4.964058] [drm] Initialized nouveau 1.3.1 20120801 for 0000:02:00.0 on minor 0
[   30.784569] sddm-greeter[1038]: segfault at 1a ip 00007f16b336d6ce sp 00007fff1701aac0 error 4 in nouveau_dri.so[7f16b312d000+a7d000]
[   85.190414] ksplashqml[1324]: segfault at 1a ip 00007f12b315c6ce sp 00007ffd1cec66d0 error 4 in nouveau_dri.so[7f12b2f1c000+a7d000]
[   96.932346] nouveau 0000:02:00.0: fb: trapped read at 010052e000 on channel -1 [0fedf000 unknown] engine 06 [BAR] client 08 [PFIFO_READ] subclient 01 [IN] reason 0000000b [VRAM_LIMIT]
[   99.523017] sddm-greeter[1546]: segfault at 1a ip 00007f45049696ce sp 00007ffe8b766630 error 4 in nouveau_dri.so[7f4504729000+a7d000]
[  110.586924] sddm-greeter[1589]: segfault at 1a ip 00007fb8d0f346ce sp 00007ffcf758d840 error 4 in nouveau_dri.so[7fb8d0cf4000+a7d000]