nv50: add 3d texture tiling and mip-mapping

nv50: add 3d texture tiling and mip-mapping

Mip-mapped 3D textures are not arrays of 2D layers
with a mip-map layout like 2D textures, therefore we
cannot use image_nr == depth for them.

Making use of "volume tiling" modes now, the allowed
modes are 0xZY where Z <= 5 and y <= 5.