intel: Refactor intel_miptree_copy_teximage()

intel: Refactor intel_miptree_copy_teximage()

Extract the body of the inner loop into a new function,
intel_miptree_copy_slice().

This is in preparation for adding support for separate stencil and HiZ to
intel_miptree_copy_teximage(). When copying a slice of a depthstencil
miptree that uses separate stencil, we will also need to copy the
corresponding slice of the stencil miptree. The easiest way to do this
will be to call intel_miptree_copy_slice() recursively. Analogous
reasoning applies to copying a slice of a depth miptree with HiZ.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>