I let it ran and got those messages:

[  417.536352] [TTM] Failed to find memory space for buffer 0xffff9e44c2076c00
eviction
[  417.544122] [TTM] No space for ffff9e44c2076c00 (1366 pages, 5464K, 5M)
[  417.550739] [TTM]   placement[0]=0x00070002 (1)
[  417.555265] [TTM]     has_type: 1
[  417.558577] [TTM]     use_type: 1
[  417.561889] [TTM]     flags: 0x0000000A
[  417.565718] [TTM]     gpu_offset: 0x00000000
[  417.569984] [TTM]     size: 131072
[  417.573383] [TTM]     available_caching: 0x00070000
[  417.578255] [TTM]     default_caching: 0x00010000
[  417.582957] nouveau 0000:08:00.0: Xorg[710]: fail ttm_validate
[  417.588786] nouveau 0000:08:00.0: Xorg[710]: validating bo list
[  417.594702] nouveau 0000:08:00.0: Xorg[710]: validate: -12
[  417.735888] nouveau 0000:08:00.0: DRM: GPU lockup - switching to software
fbcon

The screen was black and had a flashing, white underscore.

Then i tried switching to tty and that worked. After switching to graphical
screen also that was in a broken way back again. This switching around have
produces those logs:

[  867.565080] [TTM] Failed to find memory space for buffer 0xffff9e44c2279400
eviction
[  867.572841] [TTM] No space for ffff9e44c2279400 (1366 pages, 5464K, 5M)
[  867.579450] [TTM]   placement[0]=0x00070002 (1)
[  867.583972] [TTM]     has_type: 1
[  867.587279] [TTM]     use_type: 1
[  867.590583] [TTM]     flags: 0x0000000A
[  867.594409] [TTM]     gpu_offset: 0x00000000
[  867.598670] [TTM]     size: 131072
[  867.602061] [TTM]     available_caching: 0x00070000
[  867.606928] [TTM]     default_caching: 0x00010000
[  867.611626] nouveau 0000:08:00.0: supertuxkart[1075]: fail ttm_validate
[  867.618230] nouveau 0000:08:00.0: supertuxkart[1075]: validating bo list
[  867.624919] nouveau 0000:08:00.0: supertuxkart[1075]: validate: -12

The RAM size of the card is 256MB.