drm/nouveau/imem/nv50: support pinning objects in BAR2 and returning address

Various structures are accessed by the GPU through BAR2 for some reason
on newer GPUs.  This commit makes it more convenient to handle.

Will be used for GP100- fault buffers, and GV100- fault method buffers.

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