i965: Don't relayout a texture just for baselevel changes.

i965: Don't relayout a texture just for baselevel changes.

As long as the baselevel, maxlevel still sit inside the range we had
previously validated, there's no need to reallocate the texture.

I also hope this makes our texture validation logic much more obvious.
It's taken me enough tries to write this change, that's for sure.  Reduces
miptree copy count on a piglit run by 1.3%, though the change in amount of
data moved is much smaller.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>