drm/nouveau/gpuobj: type-safe accessor macros

These require an explicit struct nvkm_gpuobj pointer, unlike the previous
macros which take a void *, and work with any nvkm_object.

New semantics require acquiring/releasing a gpuobj before accessing them,
which will be made use of in later patches to greatly reduce the overhead
of accesses, particularly when a direct mmio mapping of the object is not
available (suspend/resume, out of ioremap() space, and on GK20A).

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