drm/nouveau: remove no_vm/mappable flags from nouveau_bo

'mappable' isn't really used at all, nor is it necessary anymore as the
bo code is capable of moving buffers to mappable vram as required.

'no_vm' isn't necessary anymore either, any places that don't want to be
mapped into a GPU address space should allocate the VRAM directly instead.

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