intel: Simplify and touch up the FBO completeness test.

intel: Simplify and touch up the FBO completeness test.

Now that we have miptrees for everything, we can more easily test for
!has_separate_stencil completeness.  Also, test for whether the
stencil rb is the wrong kind of format for separate stencil, or if we
are trying to do packed to different images of a single miptree.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>