drm/nouveau/disp/nv50-: split chid into chid.ctrl and chid.user

[ Upstream commit 4391d7f5c79a9fe6fa11cf6c160ca7f7bdb49d2a ]

GP102/GP104 make life difficult by redefining the channel indices for
some registers, but not others.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>