tu: Fix buffer compressed pitch calculation with unaligned sizes

tu: Fix buffer compressed pitch calculation with unaligned sizes

We can just set the extent and not bufferRowLength/bufferImageHeight,
and the extent may not be a multiple of the block size if it covers the
entire image. In this case we have to first divide to get the
width/height in terms of blocks, and then multiply by the block size to
get the buffer's pitch and layer size. Multiplying and dividing instead
won't get the correct result when the extent covers the entire image and
isn't a multiple of the block size. This also makes the code easier to
follow because we don't calculate a pitch in non-sensical units (bytes
times the block width) as an intermediate step.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5098>