drm/nouveau: teach nouveau_bo_pin() how to force a contig vram allocation

We have the ability to move buffers around in the kernel if necessary,
and should probably use it rather than failing if userspace passes us
a non-contig buffer for a plane.

The NOUVEAU_GEM_TILE_NONCONTIG flag from userspace will become a mere
initial placement hint once all the relevant paths have been updated.

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