llvmpipe: get 3D texture image stride from an array rather than computing it

llvmpipe: get 3D texture image stride from an array rather than computing it

This fixes broken 3D texture indexing when the height of the 3D texture
was less than 64 (the tile size).  It's simpler to pass this as an array
(as we do with the row stride) than to compute it on the fly.