llvmpipe: fix lp_resource_copy using more than one 3d slice

llvmpipe: fix lp_resource_copy using more than one 3d slice

These used to be illegal a very long time ago, then for some more time
nothing really emitted these so this code path wasn't hit.
Just trivially iterate over box->depth.
(Might be worth refactoring at some point since nowadays all the code
doesn't really do much except for depth textures.)

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=61093

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>