i965: Move the DRIimage -> miptree code to intel_mipmap_tree.c

i965: Move the DRIimage -> miptree code to intel_mipmap_tree.c

This is mostly a direct port.  The only bit of refactoring that was done
was to make creating a planar miptree be an early return from the
non-planar case.  Alternatively, we could have three functions: two
helpers and a main function to just call the right helper.  Making the
planar case an early return seemed cleaner.

Reviewed-by: Chad Versace <chadversary@chromium.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>