Comment # 24 on bug 46557 from Emil Velikov

(In reply to comment #23)
> here is the output for nouveau from dmesg
> 
> ...
> [    1.098755] nouveau 0000:00:05.0: >setting latency timer to 64
> ...
> [   67.077190] nouveau E[     DRM] fail ttm_validate
> [   67.077198] nouveau E[     DRM] validate vram_list
> [   67.077208] nouveau E[     DRM] validate: -12 (ENOMEM)
> ...

You have allocated only 32MB of RAM for the GPU

Try bumping it to 128 or 256MB it should resolve your issue