nouveau: bind the 3D engine to subchannel 7 and add RING_3D

nouveau: bind the 3D engine to subchannel 7 and add RING_3D

RING_3D creates a method start for subchannel 7.

Bind the 3D engine to a fixed subchannel to make it work

This is much faster than the old BEGIN_RING, since we don't need
to waste cycles trying to "autobind" stuff, when a fast static binding
is perfectly good.

Subchannel 7 is chosen because the kernel takes up the lowest ones.