drm/nvc0-nve0/graph: rename dev to priv, no code changes

There's a *lot* of code in here, and it's all going to use the PGRAPH priv
pointer rather than drm_device after the engine rework.  This is handling
all the rename-only parts of the change.
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>