drm/nouveau: detect vram amount once, and save the value

drm/nouveau: detect vram amount once, and save the value

As opposed to repeatedly reading the amount back from the GPU every
time we need to know the VRAM size.

We should now fail to load gracefully on detecting no VRAM, rather than
something potentially messy happening.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>