Necessary part from nouveau_channel_new to get a clean screan are:
- nouveau_channel_ind
- nouveau_channel_init, but only the beginning:
  - vram creation
  - gart creation
  - dma variables initialisation

There are some MEM_CACHE errors, but at least it boosts and screen is clean.

I found out that nouveau_abi16_ioctl_channel_alloc was also calling
nouveau_channel_new, but with other arguments for vram and gart, and it is
called whether or not acceleration is enabled.
Is there a specific reason to call at first nouveau_channel_new only when
acceleration is enabled, and later on when starting the GUI, to always call it?