drm/nouveau: do not allow negative sizes for now

The API allows up to 64-bits allocations, but size is handled as int
inside nouveau almost everywhere. Until this is fixed it's better to
prevent negative sizes.

The 256 kB before INT_MAX is paranoia, because of the large page
aligning below that could flip it above INT_MAX.

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