i965: Fix infinite loop regression in intel_miptree_all_slices_resolve.

i965: Fix infinite loop regression in intel_miptree_all_slices_resolve.

Commit 0ed11e333147e280208d9d0b3ff3f39970547643 fixed a "use after free"
bug by getting the next pointer before deleting the current node.

Unfortunately, it also made "next" never get updated if i->need != need.

Fixes infinite loops in piglit tests fbo-depth-array and fbo-depthtex.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>