nv50: fix textures with block size != cpp

nv50: fix textures with block size != cpp

First, using width * block size as pitch is evidently
wrong if a block contains more than 1 texel.

For tiled textures, since a block occupies a contiguous
area of memory, y addressing in m2mf has to be done by
block index, not the y coordinate itself.

This should fix compressed textures.