gallivm,draw,llvmpipe: use base ptr + mip offsets instead of mip pointers

gallivm,draw,llvmpipe: use base ptr + mip offsets instead of mip pointers

This might have a slight overhead but handling mip offsets more like
the width (and image) strides should make some things easier (mip level
being just part of the offset calculation) later.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>