drm/nouveau: check kind validity against mmu object

This is already handled in the top-level gem_new() ioctl in another manner,
but this will be removed in a future commit.

Ideally we'd not need to check up-front at all, and let the VMM code handle
error checking, but there are paths in the current BO management code where
this isn't possible due to map() not always being called during BO creation,
and map() calls not being allowed to fail during buffer migration.

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