drm/nouveau: fix command submission to use vmalloc for big allocations

I was getting a order 4 allocation failure from kmalloc when testing some
game after a few days uptime with some suspend/resumes.

For big allocations vmalloc should be used instead.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>