drm/nv40/gr: rewrite/split context takedown functions

It's completely pointless to save the PGRAPH context when destroying a
channel, so don't bother.

This commit should also fix kernel.org bug 39422, where the DRM channel
state was incorrectly being saved because we left PGRAPH FIFO access
enabled while running the ctxprog.

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