nouveau: Fix nv20-40 swizzled miptree RTs

nouveau: Fix nv20-40 swizzled miptree RTs

I just coded a patch that does this and seems to work fine. It must be
fixed since it breaks OpenGL (or the state tracker can be changed, but
it seems better to do it in the driver).

The patch also fixes NV20 and NV30 in the same way. They compile but
are untested.

I would guess that using the 3D engine is faster for the larger
levels, but the 2D engine is faster for the smaller ones (and lacks
this issue).